Basically, the general idea is that in the time leading up to
WWI, Germany knew that it wasn't on the best of terms with either
Russia or France. It's pretty obvious that both of them are gonna go for the big one. So hey, piece o cake, Germany's the greatest military power in
Europe, right? But, one small tiny eensy weensy problem - as my colleague
sfc points out -
two-front war. Enemies on both sides? Not preferable.
But our Herr Schlieffen (who actually died before he saw his plan totally fuck up) knows what to do! You see, I'll let you in on a little secret - Russia's really fucking big, y'see? And in addition to being big, it doesn't have the greatest infrastructure in the world. Basically, it's gonna take pretty long for Russia to mobilize its (admittedly, vast) forces. So Schlieffen decides to take advantage of this window of opportunity by going for France while the Russian Bear is still coming out of hibernation, so to speak. He can then utilize all his forces at once and go for the knockout with France without having to worry about the Eastern Front! Genius!
Ahem. Problem 1: Belgium. Belgium was a little country. Belgium didn't want any trouble. Therefore, Belgium would happily allow Germany free access, that German forces might tromp on through on the way to the Land of Mimes, right? Er, no. Belgium decides to fight back.
Now, Belgium isn't exactly keeping the kaiser awake at nights, see? It's not exactly a European heavyweight. But it does slow German momentum. Plus, it pisses the hell out of Britain, who decide that Belgium's violation is the final straw, and jump in, too.
Problem 2: the second problem, ironically, is that the Germans chickened out at the last second. While the Plan might seem pretty damn harebrained, it might have actually worked - but Germany decided to be a little more cautious, and spread the troops more evenly than initially planned. Result? Germany fails to punch right through France, Russia gets its boots on in time, and we're back where we never wanted to be in the first place: a two-front war. Better start diggin' them trenches, boys...