If you read the book "Why the Allies Won," by Richard Overy, you will get a slightly more nuanced response to your question. He argues that it was less due to the larger decisions to take on too many enemies at once and the military might of the combined allies, but had a lot more to do with:
1. The incredibly rapid improvement of the allied armies in both tactical ability as well as technological advancements. The Allies started off so far behind the Wehrmacht but quickly caught up.
2. An actual application of morality, that the combined war effort of the Allies was driven and benefitted from the idea that they were in the moral right.
3. An understanding by the Allies that WWII was a struggle over values and that it had to be won.
Of course, these are just Mr. Overy's opinions, I found the book interesting if not altogether convincing. It is difficult to argue with the factor that the Russians were willing to sacrifice millions of men to grind the German Army to a pulp on the Eastern Front. Still seems pretty big to me!
Wow, where to start? It's hard to choose just three. Let me choose some reasons that are true for both World War I and World War II;
1) Germany fought a two front war each time, which is militarily and strategically costly and foolish, and almost impossible to win
2) Germany had the weakest Allies in each war. Austria-Hungary and the Ottoman Empire in World War I, both of which were not very significant to the outcome of the war
3) In both wars Germany fought against both the United States and Russia, two vast countries which were almost impossible to conquer
What led to the defeat of Germany in WWII? Lots of things, but I'll list three.
- Hitler declared war on the United States after Pearl Harbor. He did not need to do it and it is not clear that the American people would have been in favor of war with Germany. By doing so, Hitler brought the US into the war.
- Hitler failed to invade England. By leaving them alone, he allowed there to be a staging ground where the US could, eventually, gather the soldiers and weapons to invade Europe.
- They invaded the USSR. This was too much to bite off and it led to the deaths of many soldiers and the loss of lots of weapons.
There were many events that caused Germany to eventually be defeated in World War II. (1) Hitler's double-cross concerning Germany's alliance with Russia and the resulting devastation during the winter battles (especially the repulse from Stalingrad in 1942-43) cost the Axis powers more than a million casualties. (2) Japan's decision to bomb Pearl Harbor forced the United States to enter the war, and once America entered with its massive military and monetary resources, Germany's demise was inevitable. (3) England's steadfast resistance in the early years of the war kept Germany from completing its invasion plans of the British Isles. It cost Germany heavily financially in addition to the loss of hundreds of aircraft.
To which period of time are you referring--WWI, WWII, even earlier? Please specify.