The Allies decided to invade Nazi-occupied France to open a two-front war.
how?
At the Tehran Conference, which took place in November 1943, the leaders of the Allied powers - Joseph Stalin of the Soviet Union, Franklin D. Roosevelt of the United States, and Winston Churchill of Great Britain - agreed to a series of military strategies. One important decision made at the conference was to launch a cross-channel invasion of Nazi-occupied France in order to open up a second front against Germany in Europe. This plan eventually led to the D-Day invasion on June 6, 1944.