This caused a rapid deterioration of relations with the United States and Britain. In April 1941 the United States cut off petroleum supplies to Thailand. Thailand's campaign for territorial expansion came to an end on December 8, 1941 when Japan invaded the country along its southern coastline at the Battle of Prachuab Khirikhan. The Phibun regime allowed for the Japanese to pass through the country in order to attack Burma and invade Malaya. Convinced by the Allied defeats of early 1942 that Japan was winning the war, Phibun decide to form an actual military alliance with the Japanese.