Not to stifle discussion but this thread "If Serbia started the ball rolling on the events that triggered WW1, why is Germany always portrayed as the prime evil in the war?" should give you a sufficient answer.

A couple of reasons:

1. The Austro-Hungary and Serbia affair threatened to be little more than yet another Balkans war (and this after the Second Balkans War in 1913!) which was certainly worthy of note but not enough to set a continent on fire. The trouble came in when Russia decided to back up their fellow Slavs in Serbia; Russia and A-H had been slapfighting over the power vacuum left by the collapse of Ottoman power in Southeast Europe, and Serbia being crushed would put A-H a peg up.

However, a war between Russia and Serbia on the one hand and A-H on the other would leave the balance strongly on Russia's side - the delicate political nature of A-H meant that too much pressure would spark ethnic tension and nationalist sentiment, which was already an issue.

Now Germany entered in. At this time, Austria-Hungary and Italy are the other two parts of the Triple Alliance. If Austria-Hungary gets crushed and fractured by Russia, that leaves Germany with only Italy to face the Russo-French alliance if another war erupts. The German war plan, the Schlieffen plan, for war with the Russo-French alliance called for the Germans to concentrate roughly 80% of their army in the West to achieve a quick victory over France so they could turn on and defeat Russia just as quickly.

Time was severely of the essence in this plan, because the idea was to get in quick and defeat France before Russia had time to fully mobilize its forces (Russian mobilization, working over a much larger nation, would take far longer than French or German). However, the way things were set up was that if the Germans began mobilization, the die was cast; they couldn't just dial it back, because their plans and preparations all revolved around putting into place and implementing the Schlieffen plan as fast as possible.

So on July 28, A-H declared war on Serbia. On July 29, Russia orders partial mobilization - against A-H. On the 30th, they order partial mobilization against Germany - that much is on them. However, Germany took the ball and ran with it from this point onward.

After more requests to Russia to halt their mobilization, and demands of France's neutrality that were replied to with mixed messages, Germany declared war on Russia on August 1. Now here's where Germany takes proactive measures: they invaded Luxembourg on the 2nd, declared war on France on the 3rd, and declared war on Belgium on the 4th.

Any hope of dialing the crisis back and averting war was gone the moment Germany, in keeping with its war plans, jumped the gun and attacked not only France, who had not made aggressive overtures, but also neutral Belgium and Luxembourg. Most of the cards to stop the war were in Germany's hands, and they not only failed to do so, but accelerated the process.

2. Also, after the Armistice, Austria-Hungary no longer existed. It was far more expedient for the Allies to hold Germany responsible because you can't milk money out of a non-existent country. Also, /u/paradoxpolitics is right in that Germany carried the war effort since about 1915, and most Allied anger was directed at them.

Source: The First World War, by John Keegan.

This is a good thread on the subject