The Invasion of Poland (1939) precipitated World War II. It was carried out by Nazi Germany, the Soviet Union, and a small German-allied Slovak contingent. In Poland the invasion is also known as "the September Campaign" ("Kampania wrześniowa") or "the 1939 Defensive War" ("Wojna obronna 1939 roku"). In Germany it is sometimes referred to as "the Poland Campaign" ("Polenfeldzug") or "the Polish-German War of 1939". For the German General Staff, it was codenamed "Fall Weiss," or "Case White".
The invasion of Poland marked the start of World War II in Europe, as Poland's western allies, the United Kingdom, Australia and New Zealand, declared war on Germany on 3 September, soon followed by France, South Africa and Canada, among others. The invasion began on 1 September 1939, one week after the signing of the Molotov-Ribbentrop Pact, and ended 6 October 1939, with Germany and the Soviet Union occupying the entirety of Poland. Although the United Kingdom and France declared war on Germany soon after Germany attacked Poland, very little direct military aid was provided (see Phoney War and Western betrayal).
Following a German-staged "Polish attack" on 31 August 1939, on September 1, German forces invaded Poland from the north, south, and west. Spread thin defending their long borders, the Polish armies were soon forced to withdraw eastward. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage. Polish forces then began a withdrawal southeast, following a plan that called for a long defense in the Romanian bridgehead area, where the Polish forces were to await an expected Allied counterattack and relief.
On 17 September 1939, the Soviet Red Army invaded the eastern regions of Poland in cooperation with Germany. The Soviets were carrying out their part of the secret appendix of the Molotov-Ribbentrop Pact, which divided Eastern Europe into Nazi and Soviet spheres of influence. Facing the second front, the Polish government decided the defense of the Romanian bridgehead was no longer feasible and ordered the evacuation of all troops to neutral Romania. By 1 October, Germany and the Soviet Union completely overran Poland, although the Polish government never surrendered. In addition, Poland's remaining land and air forces were evacuated to neighboring Romania and Hungary. Many of the exiles subsequently joined the recreated Polish Army in allied France, French-mandated Syria, and the United Kingdom.
In the aftermath of the September Campaign, a resistance movement was formed. Poland's fighting forces continued to contribute to Allied military operations throughout World War II. Germany captured the Soviet-occupied areas of Poland when it invaded the Soviet Union on 22 June 1941, and lost the territory in 1944 to an advancing Red Army. Over the course of the war, Poland lost over 20% of its pre-war population under an occupation that marked the end of the Second Polish Republic.
Germany had a substantial numerical advantage over Poland and had developed a significant military prior to the conflict. The Heer (army) had some 2,400 tanks organized into six panzer divisions, utilizing a new operational doctrine. It held that these divisions should act in coordination with other elements of the military, punching holes in the enemy line and isolating selected units, which would be encircled and destroyed. This would be followed up by less-mobile mechanized infantry and foot soldiers. The Luftwaffe (air force) provided both tactical and strategic air power, particularly dive bombers that disrupted lines of supply and communications. Together, the new methods were nicknamed Blitzkrieg (lightning war). Historians Basil Liddell Hart and A. J. P. Taylor conclude "Poland was a full demonstration of the Blitzkrieg theory.
Aircraft played a major role in the campaign. Bombers also attacked cities, causing huge losses amongst the civilian population through terror bombing. The Luftwaffe forces consisted of 1,180 fighter aircraft: 290 Ju 87 Stuka dive bombers, 1,100 conventional bombers (mainly He 111s and Dornier Do 17s), and an assortment of 550 transport and 350 reconnaissance aircraft. In total, Germany had close to 4,000 aircraft, all up to modern standards. A force of 2,315 aircraft were assigned to Weiss. Due to its prior participation in the Spanish Civil War, the Luftwaffe was probably the most experienced, best trained and well equipped air force in the world in 1939.
Between 1936 and 1939, Poland invested heavily in industrialization in the Central Industrial Region. Preparations for a defensive war with Germany were ongoing for many years, but most plans assumed fighting would not begin before 1942. To raise funds for industrial development, Poland sold much of the modern equipment it produced. In 1936, a National Defence Fund was set up collect funds necessary for strengthening the Polish Armed forces. The Polish Army had approximately a million soldiers, but less than half of them were mobilized by 1 September. Latecomers sustained significant casualties when public transport became targets of the Luftwaffe. The Polish military had fewer armoured forces than the Germans, and these units, dispersed within the infantry, were unable to effectively engage the enemy.
Experiences in the Polish-Soviet War shaped Polish Army organisational and operational doctrine. Unlike the trench warfare of the First World War, the Polish-Soviet War was a conflict in which the cavalry's mobility played a decisive role. Poland acknowledged the benefits of mobility but was unwilling to invest heavily in many of the expensive, unproven inventions since then. In spite of this, Polish cavalry brigades were used as a mobile mounted infantry and had some successes against both German infantry and cavalry.
The Polish Air Force (Lotnictwo Wojskowe) was at a severe disadvantage against the German Luftwaffe, although it was not destroyed on the ground early on, as is commonly believed. The Polish Air Force lacked modern fighter aircraft, but its pilots were among the world's best trained, as proven a year later in the Battle of Britain, in which the Poles played a major part.
Overall, the Germans enjoyed numerical and qualitative aircraft superiority. Poland had only about 600 modern aircraft. The Polish Air Force had roughly 185 PZL P.11 and some 95 PZL P.7 fighters, 175 PZL.23 Karaś B, 35 Karaś A, and by September, over 100 PZL.37 Łoś were produced . There were also over a thousand obsolete transport, reconnaissance and training aircraft. However, for the September Campaign, only some 70% of those aircraft were mobilised. Only 36 PZL.37 Łoś bombers were deployed. All those aircraft were of indigenous Polish design, with the bombers being more modern than fighters, according to the Ludomił Rayski air force expansion plan, which relied on a strong bomber force. The Polish fighters were a generation older than their German counterparts. The Polish PZL P.11 fighter, produced in the early 1930s, was capable of only 365 km/h (approximately 220 mi/hr), far less than German bombers; to compensate, the pilots relied on its maneuverability and high diving speed.
The Polish Navy was a small fleet comprised of destroyers, submarines and smaller support vessels. Most Polish surface units followed Operation Peking, leaving Polish ports on 20 August and escaping by way of the North Sea to join with the British Royal Navy. Submarine forces participated in Operation Worek, with the goal of engaging and damaging German shipping in the Baltic Sea, but they had much less success. In addition, many merchant marine ships joined the British merchant fleet and took part in wartime convoys.
The tank force consisted of two armoured brigades, four independent tank battalions and some 30 companies of TKS tankettes attached to infantry divisions and cavalry brigades.
Order of battle of invading forces:
In addition to Soviet territory, the Nazis were also interested in establishing a new border with Poland because the German exclave of East Prussia was separated from the rest of the Reich by the "Polish Corridor". Many Germans also wanted to incorporate the Free City of Danzig into Germany. While Danzig had a predominantly German population, the Corridor constituted land long disputed by Poland and Germany. After the Treaty of Versailles, Poland acquired the Corridor, which was part of territory taken by Prussia in Partitions of Poland. Hitler sought to reverse this trend and made an appeal to German nationalism, promising to "liberate" the German minority still in the Corridor, as well as Danzig, since the port city was under the control of the League of Nations.
Poland participated in the partition of Czechoslovakia that followed the Munich Agreement, although they were not part of the agreement. It coerced Czechoslovakia to surrender the city of Český Těšín by issuing an ultimatum to that effect on September 30, which was accepted by Czechoslovakia on October 1.
In 1938, Germany began to increase its demands for Danzig, while proposing that a roadway be built in order to connect East Prussia with Germany proper, running through the Polish Corridor. Poland rejected this proposal, fearing that after accepting these demands, it would become increasingly subject to the will of Germany and eventually lose its independence as the Czechs had. The Poles also distrusted Hitler and his intentions. At the same time, Germany's collaboration with anti-Polish Ukrainian nationalists from the Organization of Ukrainian Nationalists further weakened German credibility in Polish eyes, which was seen as an effort to isolate and weaken Poland. The British were also aware of this. On 30 March, Poland was backed by a guarantee from Britain and France, though neither country was willing to pledge military support in Poland's defense. British Prime Minister Neville Chamberlain and his Foreign Secretary, Lord Halifax, still hoped to strike a deal with Hitler regarding Danzig (and possibly the Polish Corridor), and Hitler hoped for the same. By again resorting to appeasement, Chamberlain and his supporters believed war could be avoided and hoped Germany would agree to leave the rest of Poland alone. German hegemony over Central Europe was also at stake.
With tensions mounting, Germany turned to aggressive diplomacy. On 28 April 1939, it unilaterally withdrew from both the German-Polish Non-Aggression Pact of 1934 and the London Naval Agreement of 1935. In early 1939, Hitler had already issued orders to prepare for a possible "solution of the Polish problem by military means." Another crucial step towards war was the surprise signing of the Molotov-Ribbentrop Pact on August 23, the denouement of secret Nazi-Soviet talks held in Moscow, which capitalized on France and Britain's own failure to secure an alliance with the Soviet Union. As a result, Germany neutralized the possibility of Soviet opposition to a campaign against Poland. In a secret protocol of this pact, the Germans and the Soviets agreed to divide Eastern Europe, including Poland, into two spheres of influence; the western third of the country was to go to Germany and the eastern two-thirds to the Soviet Union.
The German assault was originally scheduled to begin at 04:00 on August 26. However, on August 25, the Polish-British Common Defence Pact was signed as an annex to the Franco-Polish Military Alliance. In this accord, Britain committed itself to the defence of Poland, guaranteeing to preserve Polish independence. At the same time, the British and the Poles were hinting to Berlin that they were willing to resume discussions – not at all how Hitler hoped to frame the conflict. Thus, he wavered and postponed his attack until September 1, managing to halt the entire invasion "in mid-leap", with the exception of a few units that were out of communication, towards the south (the Nazi press announced that fanatical Slovakians were behind a cross border raid).
On August 26, Hitler tried to dissuade the British and the French from interfering in the upcoming conflict, even pledging that the Wehrmacht forces would be made available to Britain's empire in the future. In any case, the negotiations convinced Hitler that there was little chance the Western Allies would declare war on Germany, and even if they did, because of the lack of territorial guarantees to Poland, they would be willing to negotiate a compromise favourable to Germany after its conquest of Poland. Meanwhile, the number of increased overflights by high-altitude reconnaissance aircraft and cross border troop movements signalled that war was imminent.
On August 29, prompted by the British, Germany issued one last diplomatic offer, with Case White yet to be rescheduled. At midnight on August 29, German Foreign Minister Joachim von Ribbentrop handed British Ambassador Sir Neville Henderson the list of terms which would allegedly ensure peace in regards to Poland. Danzig was to be returned to Germany (Gdynia would remain with Poland), and there was to be a plebiscite in the Polish Corridor, based on residency in 1919, within the year. An exchange of minority populations between the two countries was proposed. A Polish plenipotentiary was to arrive in Berlin and accept these terms by noon the next day. The British Cabinet viewed the terms as "reasonable", except the demand for the urgent plenipotentiary, a form of an ultimatum. When Polish Ambassador Lipski went to see Ribbentrop on August 30, he announced that he did not have the full power to sign, and Ribbentrop dismissed him. It was then broadcast that Poland had rejected Germany's offer, and negotiations with Poland came to an end.
On August 30, the Polish Navy sent its destroyer flotilla to Britain, executing Operation Peking. On the same day, Marshal of Poland Edward Rydz-Śmigły announced the mobilization of Polish troops. However, he was pressured into revoking the order by the French, who apparently still hoped for a diplomatic settlement, failing to realize that the Germans were fully mobilized and concentrated at the Polish border. During the night of August 31, the Gleiwitz incident ("Polish" attack on the radio station) was staged near the German border city of Gleiwitz, in Upper Silesia. On 31 August 1939, Hitler ordered hostilities against Poland to start at 4:45 the next morning. Because of the prior stoppage, Poland managed to mobilise only 70% of its planned forces, and many units were still forming or moving to their designated frontline positions.
Poland's terrain was well suited for mobile operations when the weather cooperated – the country had flat plains with long frontiers totalling almost 5,600 kilometres (3,500 mi), Poland's long border with Germany on the west and north (facing East Prussia) extended 2,000 kilometres (1,250 mi). Those had been lengthened by another 300 kilometres (180 mi) on the southern side in the aftermath of the Munich Agreement of 1938; the German incorporation of Bohemia and Moravia and creation of the German puppet state of Slovakia meant that Poland's southern flank was exposed.
German planners intended to fully exploit their long border with the great enveloping manoeuvre of Fall Weiss. German units were to invade Poland from three directions:
All three assaults were to converge on Warsaw, while the main Polish army was to be encircled and destroyed west of the Vistula. Fall Weiss was initiated on 1 September 1939, and was the first operation of the Second World War in Europe.
The Polish defense plan, Zachód (West), was shaped by political determination to deploy forces directly at the German-Polish border, based upon London's promise to come to Warsaw's military aid in the event of invasion. Moreover, with the nation's most valuable natural resources, industry and highly populated regions near the western border (Silesia region), Polish policy centered on their protection, especially since many politicians feared that if Poland were to retreat from the regions disputed by Germany (like the Polish Corridor, cause of the famous "Danzig or War" ultimatum), Britain and France would sign a separate peace treaty with Germany similar to the Munich Agreement of 1938. In addition, none of its allies had specifically guaranteed Polish borders or territorial integrity. On those grounds, Poland disregarded French advice to deploy the bulk of their forces behind the natural barriers of the wide Vistula and San rivers, even though some Polish generals supported it as a better strategy. The Zachód plan did allow the Polish armies to retreat inside the country, but it was supposed to be a slow retreat behind prepared positions near the rivers (Narew, Vistula and San), giving the country time to finish its mobilisation, and was to be turned into a general counteroffensive when the Western Allies launched their own promised offensive.
The Polish Army's most pessimistic fall-back plan involved retreat behind the river San to the southeastern voivodships and their lengthy defence (the Romanian bridgehead plan). The British and French estimated that Poland should be able to defend that region for two to three months, while Poland estimated it could hold for at least six months. This Polish plan was based around the expectation that the Western Allies would keep their end of the signed alliance treaty and quickly start an offensive of their own. However, neither the French nor the British government made plans to attack Germany while the Polish campaign was being fought. In addition, they expected the war to develop into trench warfare much like World War I had, forcing the Germans to sign a peace treaty restoring Poland's borders. The Polish government, however, was not notified of this strategy and based all of its defence plans on promises of quick relief by their Western allies.
The plan to defend the borders contributed vastly to the Polish defeat. Polish forces were stretched thin on the very long border and, lacking compact defence lines and good defence positions along disadvantageous terrain, mechanized German forces often were able to encircle them. In addition, supply lines, were often poorly protected. Approximately one-third of Poland's forces were concentrated in or near the Polish Corridor (in northwestern Poland), where they were perilously exposed to a double envelopment — from East Prussia and the west combined and isolated in a pocket. In the south, facing the main avenues of a German advance, the Polish forces were thinly spread. At the same time, nearly another one-third of Poland's troops were massed in reserve in the north-central part of the country, between the major cities of Łódź and Warsaw, under commander in chief Marshal Edward Rydz-Śmigły. The Poles' forward concentration in general forfeited their chance of fighting a series of delaying actions, since their army, unlike some of Germany's, traveled largely on foot and was unable to retreat to their defensive positions in the rear or to man them before they were overrun by German mechanized columns.
The political decision to defend the border was not the Polish high command's only strategic mistake. Polish pre-war propaganda stated that any German invasion would be easily repelled, so that the eventual Polish defeats in the September Campaign came as a shock to many civilians, who were unprepared for such news and, with no training for such a disaster, panicked and retreated east, spreading chaos, lowering troop morale and making road transportation for Polish troops very difficult. The propaganda also had some negative consequences for the Polish troops, whose communications, disrupted by German mobile units operating in the rear and civilians blocking roads, were further thrown into chaos by bizarre reports from Polish radio stations and newspapers which often reported imaginary victories and other military operations. This led to some Polish troops being encircled or making a stand against overwhelming odds, when they thought they were actually counterattacking or would soon receive reinforcements from other victorious areas.
Following several German-staged incidents (Operation Himmler), which German propaganda used as an excuse to claim that German forces were acting in self-defense, the first regular act of war took place on 1 September 1939, at 04:40, when the German Air Force (Luftwaffe) attacked the Polish town of Wieluń, destroying 75% of the city and killing close to 1,200 people, most of them civilians. Five minutes later, the old German battleship Schleswig-Holstein opened fire on the Polish military transit depot at Westerplatte in the Free City of Danzig on the Baltic Sea. At 08:00, German troops, still without a formal declaration of war issued, attacked near the Polish town of Mokra. The battle of the border had begun. Later that day, the Germans attacked on Poland's western, southern and northern borders, while German aircraft began raids on Polish cities. The main axes of attack led eastwards from Germany proper through the western Polish border. Supporting attacks came from East Prussia in the north, and a co-operative German-Slovak tertiary attack by units (Army "Bernolak") from German-allied Slovakia in the south. All three assaults converged on the Polish capital of Warsaw.
The Allied governments declared war on Germany on 3 September; however, they failed to provide any meaningful support. The German-French border saw only a few minor skirmishes, although the majority of German forces, including eighty-five percent of their armoured forces, were engaged in Poland. Despite some Polish successes in minor border battles, German technical, operational and numerical superiority forced the Polish armies to retreat from the borders towards Warsaw and Lwów. The Luftwaffe gained air superiority early in the campaign. By destroying communications, the Luftwaffe increased the pace of the advance which overran Polish airstrips and early warning sites and causing logistical problems for the Poles. Many Polish Air Force units ran low on supplies, 98 of their number withdrew into then-neutral Romania. The Polish initial strength of 400 was reduced to just 54 by September 14 and air opposition virtually ceased.
By September 3, when Günther von Kluge in the north had reached the Vistula (some 10 kilometres from the German border at that time) river and Georg von Küchler was approaching the Narew River, Walther von Reichenau's armour was already beyond the Warta river; two days later, his left wing was well to the rear of Łódź and his right wing at the town of Kielce; and by September 8, one of his armoured corps was on the outskirts of Warsaw, having advanced 225 kilometres (140 miles) in the first week of war. Light divisions on Reichenau's right were on the Vistula between Warsaw and the town of Sandomierz by September 9, while List, in the south, was on the river San above and below the town of Przemyśl. At the same time, Guderian led his 3rd Army tanks across the Narew, attacking the line of the Bug River, already encircling Warsaw. All the German armies made progress in fulfilling their parts of the Fall Weiss plan. The Polish armies were splitting up into uncoordinated fragments, some of which were retreating while others were launching disjointed attacks on the nearest German columns.
Polish forces abandoned regions of Pomerania, Greater Poland and Silesia in the first week. The Polish plan for border defence was proven a dismal failure. The German advance as a whole was not slowed. On September 10, the Polish commander-in-chief, Marshal Edward Rydz-Śmigły, ordered a general retreat to the southeast, towards the so-called Romanian bridgehead. Meanwhile, the Germans were tightening their encirclement of the Polish forces west of the Vistula (in the Łódź area and, still farther west, around Poznań) and also penetrating deeply into eastern Poland. Warsaw, under heavy aerial bombardment since the first hours of the war, was attacked on September 9 and was put under siege on September 13. Around that time, advanced German forces also reached the city of Lwów, a major metropolis in eastern Poland. 1,150 German aircraft bombed Warsaw on September 24.
The largest battle during this campaign, the Battle of Bzura, took place near the Bzura river west of Warsaw and lasted from September 9 to September 19. Polish armies Poznań and Pomorze, retreating from the border area of the Polish Corridor, attacked the flank of the advancing German 8th Army, but the counterattack failed after initial success. After the defeat, Poland lost its ability to take the initiative and counterattack on a large scale. German air power was instrumental during the battle. The Luftwaffe's offensive broke what remained of Polish resistance in an "awesome demonstration of air power". The Luftwaffe quickly destroyed the bridges across the Bzura River. Afterward, the Polish forces were trapped out in the open, and were attacked by wave after wave of Stukas, dropping 50 kg 'light bombs' which caused huge numbers of casualties. The Polish flak positions ran out of ammunition and retreated to the forests, but were then 'smoked out' by the Heinkel He 111 and Dornier Do 17s dropping 100 kg incendiaries. The Luftwaffe left the army with the easy task of mopping up survivors. The Stukageschwaders alone dropped 388 tonnes of bombs during this battle.
The Polish government (of President Ignacy Mościcki) and the high command (of Marshal Edward Rydz-Śmigły) left Warsaw in the first days of the campaign and headed southeast, reaching Brześć on September 6. Rydz-Śmigły ordered the Polish forces to retreat in the same direction, behind the Vistula and San rivers, beginning the preparations for the long defence of the Romanian bridgehead area.
From the beginning, the German government repeatedly asked Joseph Stalin and Vyacheslav Molotov to act upon the August agreement and attack Poland from the east. Worried by an unexpectedly rapid German advance and eager to grab their allotted share of the country, Soviet forces attacked Poland on September 17. It was agreed that the USSR would relinquish its interest in the territories between the new border and Warsaw in exchange for inclusion of Lithuania in the Soviet "zone of interest". The USSR had openly supported German aggression, and Molotov stated after the Polish defeat:
Germany, which has lately united 80 million Germans, has submitted certain neighboring countries to her supremacy and gained military strength in many aspects, and thus has become, as clearly can be seen, a dangerous rival to principal imperialistic powers in Europe - England and France. That is why they declared war on Germany on a pretext of fulfilling the obligations given to Poland. It is now clearer than ever, how remote the real aims of the cabinets in these countries were from the interests of defending the now disintegrated Poland or Czechoslovakia.
By 17 September 1939, the Polish defense was already broken, and the only hope was to retreat and reorganise along the Romanian bridgehead. However, these plans were rendered obsolete nearly overnight, when the over 800,000 strong Soviet Union Red Army entered and created the Belarussian and Ukrainian fronts after invading the eastern regions of Poland in violation of the Riga Peace Treaty, the Soviet-Polish Non-Aggression Pact, and other international treaties, both bilateral and multilateral. Soviet diplomacy claimed that they were "protecting the Ukrainian and Belarusian minorities of eastern Poland in view of Polish imminent collapse. Vyacheslav Molotov delivered a speech on 17 September 1939:
Events arising out of the Polish‑German War has revealed the internal insolvency and obvious impotence of the Polish state. Polish ruling circles have suffered bankruptcy. . . . Warsaw as the capital of the Polish state no longer exists. No one knows the whereabouts of the Polish Government. The population of Poland have been abandoned by their ill‑starred leaders to their fate. The Polish state and its government have virtually ceased to exist. In view of this‑state of affairs, treaties concluded between the Soviet Union and Poland have ceased to operate. A situation has arisen in Poland which demands of the Soviet‑Government especial concern for the security of its state. Poland has become a fertile field for any accidental and unexpected contingency that may create a menace to the Soviet Union. . . . Nor can it be demanded of the Soviet Government that it remain indifferent to the fate of its blood brothers, the Ukrainians and Byelorussians [White Russians] inhabiting Poland, who even formerly were without rights and who now have been abandoned entirely to their fate. The Soviet Government deems it its sacred duty to extend the hand of assistance to its brother Ukrainians and brother Byelorussians inhabiting Poland.
Polish border defence forces in the east, known as the Korpus Ochrony Pogranicza, consisted of about 25 battalions. Edward Rydz-Śmigły ordered them to fall back and not engage the Soviets. This, however, did not prevent some clashes and small battles, such as the Battle of Grodno, as soldiers and local population attempted to defend the city. The Soviets murdered numerous Poles, including prisoners of war like General Józef Olszyna-Wilczyński. The Organization of Ukrainian Nationalists rose against the Poles, and communist partisans organised local revolts, robbing and murdering Poles. Those movements were quickly disciplined by the NKVD. The Soviet invasion was one of the decisive factors that convinced the Polish government that the war in Poland was lost. Prior to the Soviet attack from the east, the Polish military's fall-back plan had called for long-term defence against Germany in the southern-eastern part of Poland, while awaiting relief from a Western Allies attack on Germany's western border. However, the Polish government refused to surrender or negotiate a peace with Germany. Instead, it ordered all units to evacuate Poland and reorganize in France.
Meanwhile, Polish forces tried to move towards the Romanian bridgehead area, still actively resisting the German invasion. From September 17 to September 20, Polish armies Kraków and Lublin were crippled at the Battle of Tomaszów Lubelski, the second largest battle of the campaign. The city of Lwów capitulated on September 22 because of Soviet intervention; the city had been attacked by the Germans over a week earlier, and in the middle of the siege, the German troops handed operations over to their Soviet allies. Despite a series of intensifying German attacks, Warsaw—defended by quickly reorganised retreating units, civilian volunteers and militia—held out until September 28. The Modlin Fortress north of Warsaw capitulated on September 29 after an intense 16-day battle. Some isolated Polish garrisons managed to hold their positions long after being surrounded by German forces. Westerplatte enclave's tiny garrison capitulated on September 7, and the Oksywie garrison held until September 19; Hel was defended until October 2. In the last week of September, Hitler made a speech in the city of Danzig in which he said:
Poland never will rise again in the form of the Versailles treaty. That is guaranteed not only by Germany, but also… Russia.Despite a Polish victory at the Battle of Szack, after which the Soviets executed all the officers and NCOs they had captured, the Red Army reached the line of rivers Narew, Western Bug, Vistula and San by September 28, in many cases meeting German units advancing from the other direction. Polish defenders on the Hel peninsula on the shore of the Baltic Sea held out until October 2. The last operational unit of the Polish Army, General Franciszek Kleeberg's Samodzielna Grupa Operacyjna "Polesie", surrendered after the four-day Battle of Kock near Lublin on October 6, marking the end of the September Campaign.
Altogether, the civilian losses of Polish population amounted to about 150,000, while German civilian losses amounted to roughly 3,250 (including 2,000 who died fighting Polish troops as members of a fifth column).
Poland's defeat was the inevitable outcome of the Warsaw government's illusions about the actions its allies would take, as well as of its over-estimation of the Polish Army's ability to offer lengthy resistance|20px|20px|Erich von Manstein
Poland was divided among Nazi Germany, the Soviet Union, Lithuania and Slovakia. Nazi Germany annexed parts of Poland, while the rest was governed by the so-called General Government. On September 28, another secret German-Soviet protocol modified the arrangements of August: all Lithuania was to be a Soviet sphere of influence, not a German one; but the dividing line in Poland was moved in Germany's favor, to the Bug River.
Even though water barriers separated most of the spheres of interest, the Soviet and German troops met on numerous occasions. The most remarkable event of this kind occurred at Brest-Litovsk on September 22. The German 19th Panzer Corps under the command of Heinz Guderian had occupied the city, which lay within the Soviet sphere of interest. When the Soviet 29th Tank Brigade under the command of S. M. Krivoshein approached, the commanders negotiated that the German troops would withdraw and the Soviet troops would enter the city saluting each other. At Brest-Litovsk, Soviet and German commanders held a joint victory parade before German forces withdrew westward behind a new demarcation line. Just three days earlier, however, the parties had a more hostile encounter near Lviv, when the German 137th Gebirgsjägerregimenter (mountain infantry regiment) attacked a reconnaissance detachment of the Soviet 24th Tank Brigade; after a few casualties on both sides, the parties turned to negotiations. The German troops left the area, and the Red Army troops entered L'viv on September 22.
About 65,000 Polish troops were killed in the fighting, with 420,000 others being captured by the Germans and 240,000 more by the Soviets (for a total of 660,000 prisoners). Up to 120,000 Polish troops escaped to neutral Romania (through the Romanian Bridgehead) and Hungary, and another 20,000 to Latvia and Lithuania, with the majority eventually making their way to France or Britain. Most of the Polish Navy succeeded in evacuating to Britain as well. German personnel losses were less than their enemies (~16,000 KIA).
Neither side—Germany, the Western Allies or the Soviet Union—expected that the German invasion of Poland would lead to a war that would surpass World War I in its scale and cost. It would be months before Hitler would see the futility of his peace negotiation attempts with Great Britain and France, but the culmination of combined European and Pacific conflicts would result in what was truly a "world war". Thus, what was not seen by most politicians and generals in 1939 is clear from the historical perspective: The Polish September Campaign marked the beginning of the Second World War in Europe, which combined with the Japanese invasion of China in 1937 and the Pacific War in 1941, formed the cataclysm known as World War II.
The invasion of Poland led to Britain and France to declare war on Germany on September 3. However, they did little to affect the outcome of the September Campaign. This lack of direct help led many Poles to believe that they had been betrayed by their Western allies.
On 23 May 1939, Adolf Hitler explained to his officers that the object of the aggression was not Danzig, but the need to obtain German Lebensraum and details of this concept would be later formulated in the infamous Generalplan Ost. The blitzkrieg decimated urban residential areas, civilians soon became indistinguishable from combatants, and the forthcoming German occupation (General Government, Reichsgau Wartheland) was one of the most brutal episodes of World War II, resulting in over 6 million Polish deaths (over 20% of the country's total population, and over 90% of its Jewish minority) – including the mass murder of 3 million Poles in extermination camps like Auschwitz, in concentration camps, and in numerous ad hoc massacres, where civilians were rounded up, taken to a nearby forest, machine-gunned, and then buried, whether they were dead or not.
The Red Army occupied the Polish territories with mostly Ukrainian and Belarusian population. The Soviets, met at the beginning as liberators by local people, soon started to assert their political control in the area. This led to a powerful anti-Soviet resistance in the West Ukraine. Soviet occupation between 1939 and 1941 resulted in the death or deportation of over a million or former Polish citizens, when all who were deemed dangerous to the Soviet regime were subject to sovietization, forced resettlement, imprisonment in labour camps (the Gulags) or murdered, like the Polish officers in the Katyn massacre. Part of these casualties were retributions for the attacks of Ukrainian nationalists on the Polish villages in the West Ukraine, where vengeful feeling was particularly strong. Soviet repression also took place after the Red Army drove the German forces out of Eastern Poland in 1944, with events like the persecution of the Home Army soldiers and execution of its leaders (Trial of the Sixteen).
The exact number of people deported in the period 1939–1941 remains unknown, and estimates vary from between 350,000 and (old WWII estimates by the Polish Underground State) over two million. The first figure is based on NKVD records and does not include the roughly 180,000 prisoners of war in Soviet captivity. Most modern historians estimate the number of all people deported from areas taken by the Soviet Union during this period at between 800,000 and 1,500,000. For example, Rummel estimates the number at 1,200,000 and Kushner and Knox at 1,500,000. Bernd Wegner quotes Norman Davies's estimate that half of an approximately one million deported Polish citizens were dead by the time the Sikorski-Mayski Agreement was signed in 1941.