On September 1, 1939, Polish militiamen defended the building for some 15 hours against assaults by the SS Heimwehr Danzig (SS of the city Danzig) and special units of Ordnungspolizei (Danzig police). All but four of the defenders who escaped from the building during the surrender were sentenced to death by a Nazi German court as partisans and executed on October 5 1939.
The Polish Post Office (Poczta Polska) in the Free City of Danzig was created in 1919 under the Treaty of Versailles, and its buildings were considered extraterritorial Polish property. The Polish Post Office in Danzig comprised several buildings. In 1930 the "Gdańsk 1" building on Heweliusz Square in the Old Town became the primary Polish post office, with a direct telephone line to Poland. In 1939 it employed slightly over 100 people. Some employees at the Polish Post Office belonged to a self-defence and security organization, and many were also members of the Polish Związek Strzelecki (Riflemen's Association). According to the testimony of Edmund Charaszkiewicz, the Polish Post Office was from 1935 an important component of the Polish Intelligence organization, "Group Zygmunt."
As tensions between Poland and Germany grew, in April 1939 the Polish High Command detached engineer and Army Reserve Sublieutenant Konrad Guderski to the Baltic Sea coast. With Alfons Flisykowski and others, he helped organize the official and volunteer security staff at the Polish Post Office in Danzig, and prepare them for eventual hostilities. In addition to training the staff, he prepared the defenses in and around the building: nearby trees were removed, and the entrance was fortified. In mid-August, ten additional employees were sent to the post office from Polish Post offices in Gdynia and Bydgoszcz (mostly reserve non-commissioned officers).
In the building of the Polish post on 1 September there were 57 people: Konrad Guderski, 42 local Polish employees, 10 employees from Gdynia and Bydgoszcz, and the building keeper with his wife and 10-year old daughter (who lived in the building). Polish employees had a cache of weapons: 40 pistols, three Browning wz.1928 light machine guns and three chests of hand grenades. The Polish defence plan assigned the defenders the role of keeping Germans from the building for 6 hours, when a relief force from Armia Pomorze was supposed to secure the area.
The German attack plan, devised in July 1939, devised that the building defenders would be stormed from two directions. A diversionary attack was to be carried out at the front entrance, while the main force would break through the wall from the neighbouring Work Office and attack from the side.
The first German attack, from the front, was repelled, although Germans managed to break through the entrance and briefly enter the building (at the cost of two killed and seven wounded attackers, including one group leader). The second attack, from the Work Office, was also repelled. The commander of Polish defence, Konrad Guderski, died during that second attack from the blast of his own grenade which stopped the Germans who broke through the wall.
At 1100 German units were reinforced with two 75 mm artillery pieces, but the second attack, even with the artillery support, was again repulsed. At 1500 Germans declared a two hour ceasefire and demanded that Polish forces surrender, which they refused. In the meantime, Germans received additional reinforcements: a 105 mm artillery piece, and a unit of sappers, which dug under the walls and prepared a 600 kg explosive device. At 1700 the bomb was set off, collapsing part of the wall, and German forces under the cover of three artillery pieces attacked again, this time capturing most of the building with the exception of the basement.
At 1800 Germans brought automatic pumps, gasoline tanks and flamethrowers, which they used to flood the basements with burning gasoline. After three Poles were burned alive (bringing the total Polish casualties to six killed in action), the rest decided to capitulate. The first two people to leave the building — director Dr. Jan Michoń, carrying a white flag, and commandant (naczelnik) Józef Wąsik — were shot by the Germans (according to one version, Dr. Michoń was attacked with a flamethrower). The rest of the Poles were allowed to surrender and leave the burning building. Six people managed to escape from the building, although two of them were captured the following days.
16 wounded prisoners were sent to the Gestapo hospital, where six subsequently died (including the 10-year old Erwina). The other 28 were first imprisoned in the police building, and after a few days sent to Victoriaschule, where they were interrogated and tortured, together with some 3,000 other Polish inhabitants of Danzig.
All the prisoners were put on trial: first the 28 imprisoned in Victoriaschule on 8 September, and then the 10 who recovered in the hospital, on 30 September, without any defence lawyer. All were sentenced to death for being partisans, under the German special military penal law of 1938. The sentence was demanded by the prosecutor Hans Giesecke, declared by presiding judge Kurt Bode, and signed by General Walther von Brauchitsch, after just few hours. (A similar fate awaited 11 Polish railway workers south of the city after they foiled a German attempt to use an armored train, and were executed by a SA unit along with their immediate families.)
The prisoners were mostly executed by firing squad led by SS-Sturmbannführer Max Pauly (later commandant of the Neuengamme concentration camp) on 5 October. One, Leon Fuz, was later recognised and murdered in the Stutthof concentration camp in November. Four postmen who managed to escape and hide survived the war.
In Poland, the whole episode has become one of the better known episodes of the Polish September Campaign and it is usually portrayed as a heroic story of David and Goliath proportions. In this view, it was a group of harmless postmen who held out against German SS troops for almost an entire day. In 1979 in the People's Republic of Poland a Defenders of the Polish Post Monument was unveiled in Gdańsk.