Following the 1938 "Anschluss" between Nazi Germany and Austria, Yugoslavia came to share its northwestern border with Germany and fell under increasing pressure as its 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 Royal 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. Yugoslavia was by then almost completely surrounded by the Axis powers and their satellites, and its 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, German dictator Adolf Hitler began placing heavy pressure on Yugoslavia to join the Axis. On 25 March 1941, after some delay, the Royal 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. They placed his teenage nephew Peter on the throne and brought to power a "government of national unity" led by the head of the Royal Yugoslav Air Force, General Dušan Simović. The coup enraged Hitler, who immediately ordered Yugoslavia's invasion, which commenced on 6 April 1941.