Following the 1938 annexation of Austria by Nazi Germany, Yugoslavia shared a border with the Third Reich and came under increasing pressure as her neighbours became aligned with the Axis powers. In April 1939, Yugoslavia gained a second frontier with Italy when that country invaded and occupied neighbouring Albania. At the outbreak of World War II, the Yugoslav government declared its neutrality. Between September and November 1940, Hungary and Romania joined the Axis, and Italy invaded Greece from Albania. From that time, Yugoslavia was almost completely surrounded by Axis powers and their satellites, and her neutral stance toward the war came under tremendous pressure. In late February 1941, Bulgaria joined the Axis. The next day, German troops entered Bulgaria from Romania, almost closing the ring around Yugoslavia. With the aim of securing his southern flank before the pending attack on the Soviet Union, Adolf Hitler began placing heavy pressure on Yugoslavia to join the Axis powers. The Yugoslav government conditionally signed the Tripartite Pactthe instrument that constituted the Axisafter some delay, on 25 March 1941. Two days later, a group of pro-Western Serb-nationalist air force officers deposed Prince Paul in a bloodless coup d'état. The conspirators declared 17-year-old Prince Peter of age and brought to power a government of national unity led by General Dušan Simović. The coup enraged Hitler, who ordered the invasion of Yugoslavia which commenced on 6 April 1941.