Matilda II tanks, new Crusader tanks and 24,000 tons of oil for operations in North Africa. They were completed on 12 May. I., II., and III.; StG 1 made a determined effort against Tiger and Malta without result. The Axis air forces maintained air superiority; Hitler ordered Fliegerkorps X to protect Axis shipping, prevent Allied shipping passing through the central Mediterranean and neutralise Malta as an Allied base. Around 180 German and 300 Italian aircraft carried out the operation, and the RAF struggled to fly more than six or eight fighter sorties. Occasionally, 12 Hurricanes were flown in from British carriers but the replacements were soon used up. By mid-May, the central Mediterranean was again closed to Allied shipping and the DAK in North Africa was able to receive reinforcements, only 3% of its supplies, personnel and equipment being lost en route. From 11 April – 10 May, 111 Axis raids were carried out against military installations on Malta. Most of the heavy equipment in Grand Harbour was destroyed and the dry-docks could only be operated by hand. Efficiency of most workshops was reduced to 25% – 50%. During the first four months of German operations, the Luftwaffe dropped 2,500 tons of high explosives on Malta. It was many more times the tonnage dropped by the Italians, but far short of the amount dropped the following year. More than 2,000 civilian buildings were destroyed as opposed to only 300 during the Italian siege. Civilian casualties were low, and after the bombing of HMS Illustrious most civilians moved to safer surroundings in the countryside; by May 1941, nearly 60,000 people had left the cities, some 11,000 people ( or 66% of the population) leaving Valletta. The British had concentrated on protecting military targets and few shelters were available for civilians. Eventually, 2,000 miners and stonemasons were recruited to build public shelters but the pay was poor and the miners threatened to strike, and were threatened with conscription into the army. The workers capitulated but instituted a go-slow, trebling the cost of the work. German withdrawal In April, Hitler was forced to intervene in the Balkans which led to the campaign of that name; it was also known as the German invasion of Yugoslavia and included the Battle of Greece. The subsequent campaign and the heavy German losses in the Battle of Crete convinced Hitler that air drops behind enemy lines, using