When Hitler invaded Russia, Germany faced a war on two fronts, making success far less likely. FDR only got the US involved after Russia was attacked. Hitler had previously a non-aggression pact with Russia, allowing Hitler to focus on Britain primarily.
The US "won" in Europe mostly by allowing Japan to temporarily win in the Pacific. FDR ordered MacArthur to retreat and allow Japan to occupy the Philippines, which was an American territory at the time. Eisenhower was too risk averse, and won mostly due to Gen. Patton. Patton was the Allies' most aggressive general, most respected & feared by the German military. We also won when Germany's best general, Rommel committed suicide after charged with treason.
Japan never would have surrendered until the A bombs were dropped. Still what really made them surrender was Russia violating its non-aggression treaty with Japan. Japan preferred to lose to the US than to Russia.
Tokyo Rose also was credited with providing better intelligence to our Pacific forces than Washington ever did. She, under the nose of the Japanese, would broadcast when Japan would attack. She would say over the radio, "Tonight GI you die" which kept everyone on high alert, awaiting the "surprise" attack which was no longer a "surprise."