Eben-Emael was a Belgian fortress between Liege and Maastricht, near the Albert Canal, defending the Belgian-German border. Constructed in 1931-1935, it was reputed to be impregnable. But on 10 May 1940, 74 paratroopers of the German 7th Flieger later 1st Fallschirmspringer Division landed on the fortress with gliders (type DFS 230). One day later, they were reinforced by the German 151st Infantry Regiment. At 13:30 h on 11 May, the fortress surrendered. 1200 Belgian soldiers were captured.
Eben-Emael, a diamond-shaped fort, was Belgium's hope to defend the eastern side from invasion, charged with defending or destroying three key bridges. It also gave protection to the south what was called the Gap of Vise. A fortress to protect this approach to Liege had been conceived in the latter 19th century, but only became politically convincing after the Albert Canal was dug (to provide a route for Belgian river transport that did not require entering Dutch territory). Thus the fortress was only completed in 1935, being sited between the river and the canal that bypassed it. With its steel and concrete cupolas, Fort Eben-Emael was thought to be impenetrable.
However, the Germans had planned the capture of the fort well in advance. In preparation they had practiced assaulting forts in occupied Czechoslovakia similar to Eben-Emael. Adolf Hitler himself conceived of a plan to take over the fort by getting men on the fort by using gliders (it would have been difficult and messy to parachute a large number of men into the small area) and utilizing top secret new shaped charge (also called "hollow charge") bombs to penetrate the cupolas.