In September 1939 Germany invaded Poland and so did the USSR. These two
states divided Poland between them.
Many Poles escaped from Poland and found their way to Britain where they
served in the armed forces. Leading Poles also set up a Polish Government
in Exile in London.
In 1941 Hitler attacked the USSR and conquered the rest of Poland and much
Soviet territory. By 1944 the Soviets were retaking their own lands and moving
into Poland on their way to Germany.
This raised the question of what would happen to Poland. The key issues
concerned where Poland's borders would be, and how Poland would be run.
The Polish Government in Exile were very suspicious of the USSR. They were
reluctant to agree any terms with Stalin. They also felt that Stalin would
not keep his word even if he did give promises to Poland.