There is a reason why folks look for explanations for certain events, like WWI--such events do not make sense to them. I do not blame them in the case of WWI, a war that appears to have been started by leaders afflicted by the same mental illness. I like to liken events like the WWI to tremors and earthquakes that are mere manifestations of primary causes, such as the shifting of earth's plates. I suspect that the reason was the changeover from the agricultural to industrial age.
You must read Kissinger's "Diplomacy". It was all a matter of balance of power. The various powers set up the dominoes and all it too was one good hard blow to send them all toppling.
France was still less than pleased with that whole Franco-Prussian war thing, but Bismarck was the king of realpolitik, and kept France isolated. England and the US were plenty happy to be isolationist with England's disdain for things on the continent and the US's bizarre exceptionalism. Germany and Russia had a nonaggression pact which kept them out of the game, and Austria knew that they were Germany's prison currency so did whatever the Prussians told them to. (Prusso-Austrian was kept them in check.) The Danes who had also been smacked around by Germany in the 1870's and by the British in the 1800-1810's. They just wanted out.
Well, Kaiser Bill was a bit of a doofus so he challenged England navally and started looking at overseas expansion (thinks Bismarck avoided) so they started getting antsy (genesisone touched on this above - despite our silliness, his first bit I quoted is factual). He let the nonaggression pact expire with Russia who then needed an ally (and France was more then happy to help!).
So you have France and Russia in the Red corner and Germany and Austria in the blue. Austria and Russia had both been eyeing the Balkans at the expense of the Sick Man of Europe. Serbia shoots Ferdi and the Germans tell the Austrians they have their back and they get ready to go all gangwar on Serbia. The Russians say they will get involved since Serbia is Orthodox (and a VERY convenient political ally in the Balkans) and the Austrians are cool with that b/c they got the Germans behind them.
France tells Russia that if they still want to go at it, the Frogs will have their back, and now you have a clusterfudge of diplomacy. England still wants to stay out but says if Germany attacks Belgium they will have to come in. Germany (did I mention that Kaiser Bill was a dingleberry?) invades Belgium just like the kid whose mommy tells him that if he sings one move verse of John Jacob Jingleheimerschmidt while she's driving on icy roads in Western Nebraska...and England gets up in this. When Russia goes down they need a new ally so the Brits and Wilson fill a passenger liner to the gills with explosives and tell all the civillians to ignore that Kraut ambassador on the docks. Germans can no longer be honourable and let passengers off since the Brits have been using Q-Ships and the boat goes kablooey with the help of a torpedo.
But I digress, WWI was a perfect storm but it is very understandable when you take into account that Europe was embroiled in power politics and any nation that got isolated was toast on the international scale. Everyone tried to balance one another out with evermore entangled alliances. So it really doesn't matter whether the powder keg was lit by a Papal Canon decree or if the Serbs just gakked some Archduke for the shiites and grins of it, or if as I suggested humourously that it was an extension of the Mexican Revolution...all that matters is that the sides were balanced and everyone was itching to fight.
Right now the US has hegemonic rule (slipping away, though it be) but in the near future we are going to have a larger number of more or less evenly balanced players on the world stage and they will probably get into alliances once again just to maintain the status quo. I say, give us 50 years and we'll have another WWI-esque war.