Actually, I would say that WW1 was started by a mentality and general international stance.
The universal doctrine was to strike early and fast, before the enemy can mobilize. Basically to start with a devastating sucker-punch. That wasn't just for Germany and Austria, but really everybody. It has even been described as a "Cult Of The Offensive."
Also, since mobilization was a very disruptive thing to do, it was pretty much understood that if you do it, you use it. A lot of countries didn't even have any plans for a partial mobilization or just going on defensive allert so to speak or anything. It was an already loaded and aimed cannon, so to speak, with all that remained to be done being to pull the rope.
Every country had mobilization plans and attack plans in advance, and the doctrine to basically pull the trigger on that before the enemy can pull the trigger on his.
As you can imagine, that wasn't very conducive to diplomacy. (Not to mention that war and military threat were considered an integral part of diplomacy at the time. Whereas nowadays going "sign here or we shoot" at a neighbouring country would be considered awfully bad form, back then it was ok if you could get away with it.) But anyway, you didn't have time to sit and posture too long, lest the enemy starts his mobilization before you and catches _you_ with your pants down. Ultimatum and immediately attack was the normal doctrine for everyone.
Also, it wasn't just a case of alliances as such, but also a great game where each superpower actively countered and tried to contain, or even backstab, every other superpower. The whole 19'th century was full of examples of just that. E.g., England and France would prop the Ottomans just to counter the Russians, or England would lend its threat to help Japan smack the Russians to contain a perceived danger of Russian expansion in Asia. And then backstabbed Japan at the peace conference.
Serbia happened to be not even important as such, but something that Russia wanted and more importantly something that Russia backed militarily. So a lot of the politics and war plans of Germany and Austria had to do with just not letting Russia get it. Russia's having started a massive modernization of its army in IIRC 1912 just made it a bigger target. Both Germany and Austria were just itching to sucker punch it before it finishes that. (And before that sounds too bad, Britain and France would have punched it instead, if Germany and Austria didn't. _Everyone_ had that great game mentality, and nobody wanted a too powerful Russia.)
It's also often forgotten that WW1 was pretty much a continuation of the Balkan Wars. That area was already a powder keg, and Serbia was already trying to become an empire in the region. Being backed by Russia just made it bolder. Although the Russians did back the more peaceful government of Pasic, the military faction was still basically chest thumping and posturing at every country around, and basically trying to establish Serbia as a new world-class a-hole.
A lot of that posturing was aimed at Austro-Hungarian occupied Bosnia, which the serbian military faction thought it should belong to Serbia. Basically before condemning Austria too hard for that ultimatum, please do remember that the relationship between the two was already strained to near breaking point. Serbia was the new bully in the region, was constantly making threats as Austria-Hungary and generally was giving Austria-Hungary plenty of reason to want to put it in its place once and for all.
Also, Gavrilo Princip who assassinated the Austrian prince _was_ a Serb and trained by the Serbian militarist faction, along with the other two. Or at the very least the Austrian government was at the time convinced that those are Serbian agents, and not just some anarchistic students. And Princip himself may or may not have been an idealist and just wanting Bosnia to be free, that was not what Serbia really wanted. Serbia only wanted a Bosnia free of the Austrians so _they_ can take it.
Basically while technically the Austrian ultimatum did start it, that ultimatum didn't happen in a vaccuum. The world back then was really a very different world than right now.