Following the 1938 "Anschluss" between Germany and Austria, Yugoslavia shared a border with the Third Reich and came under increasing pressure as her neighbours aligned themselves with the Axis powers. In April 1939, Italy opened a second frontier with Yugoslavia when it 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 Tripartite Pact, aligning themselves with the Axis, and Italy invaded Greece. From that time, Yugoslavia was almost completely surrounded by the Axis powers and their satellites, and her neutral stance toward the war became strained. In late February 1941, Bulgaria joined the Pact. The following day, German troops entered Bulgaria from Romania, closing the ring around Yugoslavia. Intending to secure his southern flank for the impending attack on the Soviet Union, Adolf Hitler began placing heavy pressure on Yugoslavia to join the Axis. On 25 March 1941, after some delay, the Yugoslav government conditionally signed the Pact. Two days later, a group of pro-Western, Serbian nationalist Royal Yugoslav Air Force officers deposed the country's regent, Prince Paul, in a bloodless coup d'état, placed his teenaged nephew Peter on the throne, and brought to power a "government of national unity" led by General Dušan Simović. The coup enraged Hitler, who immediately ordered the country's invasion, which commenced on 6 April 1941.