Following World War I, the Belgians built a system of fortifications - similar to France's Maginot Line - surrounding Liège to prevent another German invasion. Fort Eben-Emael, the largest fortress ever built, anchored the network and was completed in 1935. Eben-Emael featured multiple 60, 75, and 120mm gun emplacements protected by armor plating and reinforced concrete as thick as 13 feet. Barbed wire, cliffs, anti-aircraft batteries, machine guns and additional - albeit smaller - fortifications in the surrounding area protected the underground fort.
As the Wermacht moved west toward France in May, 1940, the Germans had to capture the bridges spanning the Albert Canal intact (they were rigged to detonate), and the guns at Eben-Emael had to be neutralized.
With some 1,200 soldiers manning the technologically advanced fortress, defeating Eben-Emael would be extraordinary difficult. But Adolf Hitler himself figured that a few dozen engineers with specialized explosives landing directly on top of the structure could pull off the operation.
What followed was the world's first combat glider landing and perhaps one of the most daring raids in military history.