Air drops were infrequent. Deliveries from the west were limited by Stalin's refusal to let the planes land on Soviet territory, the low priority placed by the British on flights to Poland; and the extremely heavy losses sustained by Polish Special Duties Flight personnel. Britain and the United States attached more importance to not antagonizing Stalin than they did to the aspirations of the Poles to regain their national sovereignty, particularly after Hitler attacked the Soviet Union in June 1941 and the Soviets joined the Western Allies in the war against Germany.