my guess is:
The most pressing threat, the Allieds decided in the beginning, was Hitler’s Germany. The German army seemed to be on the brink of defeating the Soviet army, its one-time ally. If the Russians fell, Germany could turn its full attention to Britain.
Soviet dictator Josef Stalin wanted the Allies to launch an invasion of German-held Europe as soon as possible, because Russia was being mauled by the Germans. But Churchill wanted to nibble at the edges of the German empire while bombing Germany from the air, and FDR went along with the Brits.
