Why do historians call between 1919 and 1939 the 20 year truce?

It was the reaction of Marshal Ferdinand Foch when he saw the terms that Germany was forced to accept in the treaty of Versailles.

"This isn't a peace, it's a twenty year truce!"

This came about because the three victorious powers that were forcing Germany to sign the treaty, Britain France and America, wanted very different things.

The American president Woodrow Wilson wanted to create the League of Nations, rather like a beta version of the United Nations, thus he wanted to forgive Germany so that it could take its place in the League, and peaceful governing of the world could continue.

The French wanted to weaken Germany so much that they could never be a threat. You must remember that the Western Front had largely run through France, it was a miracle that she survived. 1,382,400 Frenchmen were killed and 3,594,889 were wounded terribly. 12.5% of the population of France, mostly between the ages of 20 and 35, and all of the men, would never work again and instead of adding to the economy, would drain it. The French wanted Germany to pay a ridicules amount of money to make up for the war, they wanted to make the Rhineland (the area that borders France) and make it into a separate state, and a buffer zone, and they wanted Germany to admit that the whole war was their fault.

The British wanted to see Germany punished for the war, but they didn't want them to be crippled forever. They wanted Germany to recover so that they could become a useful trading partner again.

In the end, Britain persuaded the other two powers to compromise. The treaty wasn't as harsh as the French wanted, and not as lenient as the Americans wanted. And therein lies the irony, if the Americans had had their way there wouldn't have been so much German resentment, and if the French had had their way Germany would probably have been to weak to go to war in '39. You could argue that Britain, in trying to find a compromise to fit all helped the war along. On the other hand, France and America were at each other's throats over the terms of the treaty, and if Britain hadn't helped them to reach a middle ground, things could have gone a lot worse.

Anyway, despite good intentions, three different opinions all trying to work with each other meant that sensible policies had to be abandoned and foolish ones put in just to appease the others. People quote Foch in calling 1919-1939 a twenty year truce because, in retrospect, the treaty seemed bound to break down.