Fort Breendonk was originally built by the Belgian Army in 1906–13 as part of the second ring of defenses of the National Redoubt protecting the important port-city of Antwerp. It was covered by a five-metre thick layer of soil for defense against bombings, a water-filled moat and measured . It saw military service after the German invasion of Belgium in World War I. The siege of Antwerp begun in September 1914 and Breendonk came under fire from German howitzers out of range of its own guns on 1 October. However, the Germans successfully breached the Belgian line at Lier and were able to capture the city without making an assault on Breendonk. The fort's garrison surrendered on 9 October 1914. Breendonk was used briefly in World War II but was already militarily obsolete. During the German invasion of Belgium in May 1940, it served as the initial headquarters of the Belgian General Staff under King Leopold III but was abandoned as the Germans advanced. The Belgian Army surrendered on 28 May 1940, beginning a period of German occupation which lasted until September 1944.