 Initially developed in the late 1930s by Germany, glider infantry units were used extensively during World War II but are no longer used by any modern military.With the treaty of Versailles preventing any other form of pilot training in Germany, large numbers of gliding clubs and schools were formed there after World War I.Later, when planning the invasion of France, the German military was faced with the problem of the Belgian fort of Eben Emael which dominated the River Meuse.