Abraham Lincoln said, "Public sentiment is everything. WITH public sentiment, nothing can fail; WITHOUT it, nothing can succeed."
To maintain public sentiment in your favor, you have to keep people believing that you're righteous in everything you do. If you do something wrong such as commit a crime (like in a war) you need to hide your actions as much as possible. The best way to do this is to win wars.
The Allies were in a position to win WW1 and WW2 because they had the United States and the Federal Reserve Bank on their side. The Federal Reserve Bank is an unlimited funding source for United States foreign policy. Thus, when the United States went into WW1 & 2 they had a no-limit spending policy that allowed them to stay in the war until their opponent (Germany in both cases) ran out of money. This guarantees the win and the opportunity to tell the story the way they want the world to believe it.
The ending of WW1 was fabricated as well as most of WW2. The Allies were as guilty of crimes against humanity as they accused Germany of being. The only difference is, the Allies won the war and absolved themselves of any wrongdoing by calling it "justified".