In June 1940, the Soviet Union forcibly annexed Estonia, Latvia, and Lithuania. It also seized the disputed Romanian regions of Bessarabia, Northern Bukovina, and Hertza. But after the German Army invaded the Soviet Union in Operation Barbarossa in June 1941 and declared war on the United States in December 1941, the Soviet Union and the Allied powers worked together to fight Germany. Britain signed a formal alliance and the United States made an informal agreement. In wartime, the United States supplied Britain, the Soviet Union and other Allied nations through its Lend-Lease Program. However, Stalin remained highly suspicious, and he believed that the British and the Americans had conspired to ensure that the Soviets bore the brunt of the fighting against Germany. According to this view, the Western Allies had deliberately delayed opening a second anti-German front in order to step in at the last minute and shape the peace settlement. Thus, Soviet perceptions of the West left a strong undercurrent of tension and hostility between the Allied powers.