"Of the four wars in my lifetime, none came about because the U.S. was too strong."- Ronald Reagan
"And that prince who bases his power entirely on...words, finding himself completely without other preparations, comes to ruin..." -- Niccolo Machiavelli
After the horrors of WWI, people became so adverse to war that, in 1928, nations across the world actually signed onto an international treaty called the Kellogg-Briand Pact. This, let's be generous and call it "ambitious" treaty, actually made war illegal and it was signed by 62 nations including Britain, France, the United States, and perhaps most notably, Germany.
Now, given that humankind has forever been engaged in wars of some sort or another, you may be wondering how this treaty was to be enforced? Well, there's the hitch. There was no enforcement mechanism. Everyone just signed on to the treaty and wished for the best.
That was exactly the sort of mentality that encouraged the world to stand by helplessly as Germany, Italy, and Japan ran wild and attacked their neighbors during the thirties and eventually began the biggest, bloodiest conflagration that the world had ever seen.
This should have produced a number of lessons for the human race that would stand through time immemorial. Feeding the alligator and hoping that he eats you last, i.e. appeasement, isn't a wise strategy. Strong nations are foolish if they allow weaker nations that mean them harm to gather strength until they are a danger. When the leaders of other nations tell you that they mean you harm, take them at their word. War may be tragic, but it's also necessary. You could go on and on with the lessons that we should have learned from World War II.
Fast forward to the present and the blind utopianism of the twenties and thirties is not only back with a vengeance, it's worse than ever.