German forces invaded Greece and southern Yugoslavia from Bulgaria in the first week of April 1941. Following the collapse of resistance in Yugoslavia, the left flank of the Vermion line held by Greek and British Empire forces became exposed. A new plan intended that Imperial British forces would hold off Germans forces in western Macedonia, until non-motorised Greek infantry units had withdrawn on foot from Mount Vermion to Mount Siniatsiko, and a new defensive line had been formed between Mount Olympus and the Aliakmon River.