Poland was invaded by both Nazi Germany and the Soviet Union in 1939, and its invasion marked the beginning of World War II. Great Britain and France declared war on Germany as a result of this invasion. After the invasion, some Polish fighter pilots and other military personnel who had managed to escape from the nation before it was overrun participated in many of the major engagements of the war. They were based in Great Britain, and their contributions were especially noteworthy in the Battle of Britain, when Polish pilots flew alongside RAF fighters. Polish soldiers took part in the invasion of Normandy and other battles, as well. So Poland was on the side of the Allies, even though the homeland was under occupation by Nazi Germany. Once the Soviets, driven out when the Germans invaded the USSR in 1941, returned in 1944, they were assisted by Polish partisan fighters (though some also resisted the Soviets).