The German und and Soviet attack on Poland in September 1939 lead to the arrest and imprisonment of the Polish government.  As a result, a government in exile was formed which first operated from Paris, then from Angers and later - shortly before the French surrender - moved to London. It was considered to be the legitimate government of Poland and e.g. coordinated the Polish Armed Forces in the West. Władysław Raczkiewicz became the first Polish president in exile. The government remained active after the war and officially handed over its authority after the first half-free elections in Poland in 1990.

The Potsdam Conference (July 17th - August 2nd 1945) can be seen as the continuation of the Yalta Conference after the end of the war. Among other points, the delineation of the border between Poland and Germany was determined here.