A history book will tell you that the First World War was started due to the assassination of the Archduke Franz Ferdinand, and a tangled mess of alliances and defensive treaties criss-crossing Europe. And, of course, many of those countries had been vaguely itching for a fight for a while, each feeling like their super-modern weaponry made them unstoppable.
But these seem like poor motivators for a kid from London or a husband from Berlin to go to war. In World War 2, there were many famous motivators on each side of the conflict, such as the German resentment of the Allies for their post-war treatment or the American fury after Pearl Harbor.
Maybe I'm underestimating the potency of those alliances in the mind of the common citizen, but it seems like "I'm going to enlist because one of my government's allies was attacked by one of my government's enemy's allies because some random guy from one of my government's enemy's allies' countries was shot" isn't enough to get many people to leave their wives and girlfriends.
When the boys marched off to war in 1914, or perhaps more importantly when the later boys joined the front in 1915 or 1916, what were they fired up about? Surely we have interviews or letters, or even biased propaganda, outlining the issues that the soldiers of each side considered the most important reasons for continuing the fight? The French were presumably fighting to defend their homeland and the Serbians and Austrians no doubt felt the assassination hit quite close to home, but what about the average German or British or Russian or American or Turkish or Italian soldier in the trenches? What were the issues that they felt were the most important reasons to fight?
Or were they actually just THAT fired up about "king and country"? What did contemporary chroniclers such as Erich Maria Remarque ("All Quiet on the Western Front") or Robert Graves ("Goodbye to All That") have to say?