Stalin, Roosevelt, and Stalin planned a strategy for the allies. Stalin wanted the British and the Americans to attack Germans. So the British attacked by Africa and the Americans from the east, in that way, Germans had to move and centralize more and in May 1943, Germans finally abandoned Africa.
Next, the United States and the British armies went to Italy, where they conquered Sicily. German Nazis tried to defend but it was impossible, many people died, food, cloth, gasoline, and other goods were getting over and the war continued.
By 1944, the allies did a massive play to conquer northern France. A month later, the allies had forced the Germans out of France, Belgium, Luxembourg, and much of the Netherlands.
At the same time, the Soviets were attacking Germany pushing them to the eastern. Finally, Allies attacked Berlin, so Hitler ordered his troops to have a final war with all their provisions. Germany was surrounded and a week later, Hitler killed himself and Germany Nazi empire was over. Roosevelt also passed away so he was no available to see this victory.
Japan decided to try to destroy the U.S navy, so the United States defended and Japan lost badly. In March 1945, an Island called Iwo Jima was captured and by June they had control over Okinawa, an island just 350 miles away from Japan.
The U.S military wanted to invade Japan, but Truman analyzed that it would cost half of a million lives, so they decided to throw two bombs over Japan, one on Hiroshima and the other in Nagasaki, then, in September, Japan surrounded.