When Hungary, Bulgaria and Romania joined the Tripartite pact in 1940, pressure greatly increased on Yugoslavia to join in as Hitler was trying to protect its southern flank before launching the attack on the Soviet Union. The signing of the Treaty of the Kingdom of Yugoslavia with Germany on March 25, 1941, was followed two days later by a coup led by aviation general Dusan Simovic. Regent prince Pavel was thrown out and authority was granted to young Peter. General Simovic took over the provisional administration of the government. Thus, Yugoslavia did not seem to be reliable anymore to Hitler, and so on April 6, 1941, according to the operation Marita and without a formal declaration of war, Axis forces invaded the Kingdom of Yugoslavia. The attack began with the bombing of Belgrade, killing 20,000 people. The resistance of the Yugoslav royal army was only symbolic, as only half of the recruits were able to collect due to slow mobilization, and the military equipment and doctrine of Serbia from the Balkan wars and the First World War were obsolete. Thus, on April 10, German troops have already reached Zagreb and on April 12 Belgrade. The Italian army launched its attack only on April 11, when Hungary joined. At that time, the German army was already in Karlovac. The Italian army split into two parts: it penetrated the part towards both Ljubljana and beyond via Kočevje, and the second part penetrated via Dalmatia. The German army also broke out of Bulgaria and with the mobilized units easily prevented the withdrawal of the Yugoslav army into the Thessaloniki front.