Park and the co-located 21 October Museum, and has been the subject of several poems and feature films. The anniversary of the massacre is commemorated annually in Serbia as the Day of Remembrance of the Serbian Victims of World War II. Background Encirclement and invasion of Yugoslavia Following the 1938 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 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 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 the country's invasion, which commenced on 6 April 1941. Yugoslavia was quickly overwhelmed by the combined strength of the Axis powers and surrendered in less than two weeks. The government and royal family went into exile, and the country was occupied and dismembered by its neighbours. The German-occupied territory of Serbia was limited to the pre-Balkan War borders of the Kingdom of Serbia and was directly occupied by the Germans for the key rail and riverine transport routes that passed through it, as well as its valuable resources, particularly non-ferrous metals. The occupied territory covered about and had a population of 3.8 million. Hitler had briefly considered erasing all existence of a Serbian state, but this was quickly abandoned and the