The correct answer to this open question is the following.
Although there are no options attached, we can say the following.
Allies allowed the Soviet Union to join them when they knew full well about Stalin's tactics because Great Britain and France really knew that they alone, could not stop Hitler's troops from invading France and then the British territory.
Winston Churchill and Charles De Gaulle knew that although Joseph Stalin was a tyrant and a dictator, he ruled over a great land with supplies and had the kind of strong men in its army that could be of great help to contain the Germans.
That is how, the three allies with the help of the United States army, could devise a plan to retake North Africa to invade through Italy (the Axis troops had occupied North Africa), invade France from Britain and Germany from the Soviet Union. Once done that, the United States Navy with the help of the Allies could attack and defeat Japan in the Pacific.