Serbia had an alliance with Russia. When Austria-Hungary began threatening Serbia, Russia said they would mobilize. Germany had an alliance with Ausria-Hungary, and didn't much like the idea of Russia mobilizing on their eastern frontier or threatening their ally. Germany, knowing that France had an alliance with Russia, did say that if France promised not to mobilize their forces, then they wouldn't either. France was asked repeatedly what their intentions were, but refused to give a straight answer. This left Germany with no choice but to mobilize to counter Russia and possibly France. France, seeing Germany mobilize, then dropped all pretenses and begin calling up their own forces. So fearing an attack from two sides, Germany pulled a George Bush Jr. and decided to strike first in the hope of knocking France out before the ponderous Russian juggernaut could be made combat ready. The only mistake the Germans made was deciding to go through neutral Belgium, which brought Great Britain into the war.

In my opinion, the outcome of W.W.I is the reason why Europe is as screwed up today as it is. If time travel ever does become possible, then the mistake that was W.W.I should be corrected.