As the Belgian army was ill-prepared and outnumbered by the better-armed and more numerous German troops, the Belgian army had to relinquish control of the strongholds of Liège (which fell on August 16) and Namur, which fell into the hands of the Germans on August 24.
Unable to withstand the massive German offensive, King Albert I of the Belgians instructed the army to withdraw to the "Fort of Antwerp" on August 20. This collection of fortifications and defensive positions around the city of Antwerp was considered to be the "réduit national" and impenetrable. The "Fort of Antwerp" consisted of an outer and an inner ring around the city of 19th-century forts and strongholds within a distance of several kilometers of each other, built to defend the vital harbor of Antwerp. These defenses could not withstand German siege artillery, which was specifically designed to destroy border fortresses and clear the way for the invasion of France through Belgium. This artillery included the famous "Big Bertha" guns (not to be confused with the later "Paris Gun") and heavy but mobile Austrian howitzers manned by Austrian crews.
The German Army attacked Antwerp on September 28, capturing many of the outer ring forts, including Fort Catharine. On October 1 the Belgian government sent a telegram to the British announcing that they would retreat from Antwerp in three days time. The British, who were not well-informed about the effect of the heavy guns, were shocked and on the second of October they sent the First Lord of the Admiralty Winston Churchill over to inspect the situation. He telegrammed back that Antwerp would have to be reinforced and then relieved. On the night of October 3 a brigade of British marines arrived as the first element of a Naval Division to reinforce the defenders. This was a great boost to Belgian morale. The Naval Division, however, largely consisted of untrained recruits.
October 5 was a crucial date during the Siege of Antwerp; the German army broke through the Belgian defences in the town of Lier, 20 kilometers southeast of Antwerp and moved on to the town of Dendermonde (south of Antwerp) where it attempted to cross the river Scheldt. This "pincer movement" of the German army threatened to block the western retreat route of the Belgian army out of Antwerp, its eastern and southern escape routes being blocked by German troops and its north blocked by the closed Belgian-Dutch border. The Dutch did not offer any military assistance, not wanting to be drawn into the conflict, preferring neutrality. Also, many British members of the Naval Division unintentionally retreated into the Netherlands and were interned throughout the war. This added to the embarrassment of the episode for Churchill.
The Belgian army retreated before being trapped and left the city of Antwerp to its own defenses. Belgian forces fled westwards towards the coast on October 6 eventually stopping the German advance on the banks of the river Yser. The city of Antwerp was defended by the remaining fort's garrisons. Most of these troops were abandoned by their officers and many soldiers deserted and destroyed their own weaponry and ammunitions.
One third of the Belgian Army, about 40,000 soldiers, fled north to the Netherlands, followed by one million civilian refugees in 1914. The Netherlands interned Belgian refugees as far as possible from the Belgian border, for fear of being drawn into the conflict, many continued living in the Netherlands after 1918 and never returned to Belgium.