The stupidity of Hitler is what cost the Germans during WWII, but the Russians bore the brunt of the fighting and destroyed the heart of the German military. Most of the best units of the Wehrmacht were destroyed on the Eastern front after Hitler diverted his armor south to take the oil fields instead of allowing them to take Moscow. Hitler himself snatched defeat from the jaws of victory with his ridiculous strategic orders. His biggest blunder on the Western Front was not destroying the British Army at Dunkirk and having no plan to invade Britain right away (no ships). He then assumed England would join his cause, but there was this pesky man named Churchill......
However, you are right to laud the P-51 as a decisive point in the battle and it greatly aided the Russians as well by destroying what was left of the Luftwaffe and allowing our strategic bombing campaign to be far more effective. What an incredible plane it turned out to be! Still to this day the best combat aircraft in history by any measure.
Some military guy wrote that near the end of the war, they were sending out the bombers not only to hit whatever was left, but to lure out the Luftwaffe so the P-51s and P-47s could wipe them out. They did that job very well.