Its always seemed to me that in the run-up to WW1 almost all the participating nations' rulers and military were just looking for an excuse to have a go at each other. Any semi-believable excuse would be justification enough, so one hot-headed Serb did very nicely, thank-you.
Once the war was rolling the various Empires got dragged in along with the Americans.
Hindsight shows that the Versailles Treaty was vindictive enough to virtually guarantee trouble would erupt a bit later. I've always wondered about its severity: possibly something to do with the pro-war politicians on the winning side distracting attention from their own misdeeds?