With the MG 34, the German Wehrmacht introduced an entirely new concept in automatic firepower - the general-purpose machine gun (GPMG). In itself the MG 34 was an excellent weapon: an air-cooled, recoil-operated machine gun that could run through belts of 7.92mm ammunition at a rate of 850rpm, delivering killing firepower at ranges of more than 1,000m. Yet simply by changing its mount and feed mechanism, the operator could radically transform its function. On its standard bipod it was a light machine gun, ideal for infantry assaults; on a tripod it could serve as a sustained-fire medium...

In early May 1940, the fortress of Eben Emael was a potent sentinel over the Belgian-Dutch borderlands. A ferro-concrete monster built in the 1930s on the Albert Canal, the fortress covered 75 hectares on the surface, had 5km of tunnels underground and was studded with bunkers, gun turrets and casemates. Add a garrison of 1,200 men and the natural protection of 60m-high canal walls, and Eben Emael gave the impression of near-impregnability.

Yet on 10 May, in the very first hours of Hitler's campaign in Western Europe, just 78 elite airborne soldiers managed to defeat this fortress in...