Prior to World War I, the German military was especially interested in the development of superweapons due to the need for the Schlieffen plan to march past a line of Belgian fortifications constructed specifically to stop such an invasion route. During the opening phases of the war, the Germans employed a 420 mm Krupp howitzer (the Big Bertha) and two 305 mm Skoda Mörser M. 11 mortars to reduce the famous fortresses of Liège and Namur. Their low overland mobility made them arrive later than the infantry at Liège, so several infantry assaults were made with heavy loss of life and generally little success. The guns arrived a few days later and reduced the forts at Liège one-by-one over a short period of a few days.