Victoria's golden jubilee. 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. Larger artillery after this opening period was generally limited to railway guns, which had much greater mobility, or naval monitors (two of the British Lord Clive class monitors were fitted with an 18-inch (457 mm) gun, and HMS General Wolfe fired at a railway bridge in Belgium). All of the major powers involved employed such weapons in limited numbers, typically between 280 and 305 mm (11 to 12 inches) although some larger weapons were also used. The longest-ranged and longest-barreled of the heavy guns deployed in World War I was the Paris Gun, which was used to bombard Paris from a distance of over . The gun had a bore diameter of and a barrel length of . It was fired from concealed fixed positions in the forest of Coucy. The British attempted to develop weapons to counter the Paris Gun, but none was ready for testing until after the Armistice. A 16-inch gun under development by Vickers for a class of never-built Russian battleships was converted and lined down to , with the designation "8-inch sub-calibre Mark I". The barrel was 120 calibres long. Testing commenced in February 1919, but after only six rounds were fired a crack was discovered, and the gun was scrapped in 1928. A weapon of similar concept, the "8-inch sub-calibre Mark II", was converted from a 12-inch gun (either Mark XI, XI*, or XII), producing a /75 calibre weapon. However, with the war ending before the gun was ready, this weapon was soon scrapped. Development continued during the inter-war era, although at a more limited pace as aircraft were expected to take over in the