also pushed back the Italians into Italian-occupied Albania, and, while the Greeks hesitated to ask for British troops for fear of German intervention, a few RAF squadrons participated in the operations against the Italians. Hitler was infuriated that the British were now within striking distance to the vital Romanian oil fields of Ploieşti. The Germans postponed their attack on the USSR from 15 May 1941, to the 22 June in order to secure their south-eastern flank. Germany began preparing for Operation Marita, the invasion of Greece via Bulgaria. At this point, Yugoslavia's refusal to join the Axis camp incurred Hitler's wrath. The Yugoslavian government under Regent Prince Paul had initially been in favour of joining Germany. But a coup d'état had toppled the government, deposed the Regent, and proclaimed the teenage Peter II as king. In response to this, Hitler ordered the invasion of Yugoslavia, "Operation Punishment" (Strafgericht). During the bombing of Belgrade, the center of the Yugoslavian capital was destroyed and 15,000 people killed and made homeless. The bombing started 6 April and continued for four days until 10 April. The Luftwaffe provided invaluable air support during the rapidly victorious Balkans Campaign. It quickly established absolute air superiority, allowing the German Army to carry out the conquest of Yugoslavia and Greece within only three weeks (from 6–30 April 1941). Once again the Stuka came back into its own during these campaigns. Having suffered heavy losses during operations over Britain, the Luftwaffe command realised it was vulnerable against a well-organised and determined air defense. These components were lacking in the Yugoslavian Air Force, and as a result the Stuka was able to operate effectively without fear of opposition. The Stukas took a heavy toll of Yugoslav ground and naval forces, which including the destruction of most of its torpedo-boats and sinking the 1870-ton seaplane tender Zmaj. During the very brief campaign in Yugoslavia, the Luftwaffe engaged the Yugoslavian Air Force's Do 17s and Bf 109s. The Yugoslavs had license built nearly 50 of the Do 17, but most of these were destroyed or captured. The offensive continued into Greece in which the Luftwaffe eliminated Greek and British opposition in the air, although many strong points on the ground such as Fort Istibei, and other parts of the Metaxas Line held out under relentless air assault for several days. The Battle of Crete remained to be fought. The Greek island was