WWI was caused by a bunch of treaties -- The Triple Alliance, The Triple Entente, and others -- that provided a "domino effect". Like a bunch of kids shouting "He's my friend! You can't do that to him!" and a schoolyard brawl breaks out -- only in this one millions die. Governments fall (not that they were all lily-white to begin with) and the world changes.

The Treaty of Versailles imposes government-approved looting of German resources, which pisses off the Germans, who follow the rantings of a former Wermacht corporal and blame the Jews for the problems of inflation, unemployment, and so on.

Of course, the glory of the conquered must the restored, so Germany tosses the Treaty and rearms. After taking the Rhineland, Austria, and Czechoslovakia, it makes a treaty with Russia and invades Poland.
Britain and France didn't care before, Hitler assumes they don't care now. He;s wrong and millions die. Of course, his allies, Japan and Italy, also suffer. The whole world suffers, just as it did a quarter century before.

Promises made to various countries during the war so that their support would be gained are kept, but many are broken. France, for instance, promises Indochina independence and reneges; the Vietnam war results and many, many die. Korea is divided at the 38th parallel, and Stalin back Kim Il Sung, who invades in 1950. Millions die, and as usual a country is devastated (this war has not ended and North Korea abrogated the truce a couple years ago).

It all hangs together....