Okeyyy...I feel obligated to defend my country. First of all, you can't just blame "Serbs" for World War One because it was not as if it was the Serbian army declaring war, the guy who shot Franz Ferdinand was a member of a terrorist organization called the Black Hands, which, because its not wearing a Serbian army uniform, can not be legitimately associated with a state, and therefore I think it was wrong for Austria to declare war because they declared war against a nation, when the assassin was part of a group that just happened to be based there.

Secondly, the Austro-Hungarian empire was declining fast and Serbia was actually beating them on their own at first. Sure, Russia had mobilized their troops, but they had not declared war on Austria. Then Germany declared war on Russia, turning it into a World War. A country mobilizing its troops is not a valid reason to declare war on them. They weren't even mobilizing their troops against Germany. Also, had Germany not declared war, it would not have been a World War. Its also no small secret that Germany pressured Austria to declare war.

But why did Germany declare war? In the years leading up to World War One, nationalism was on the rise, which naturally gave rise to cult of the offensive - the idea that war is good and the best way to prove your country is the best is for them to go fight and win a war. In the years leading up to World War One, Germany was a rising power that had not yet proved itself, where as Britain, France, Italy, Spain, Austro-Hungary and the Ottoman empire were either stagnant or on the decline. The best way for Germany to assert itself and to have more influence in World politics (according to the above mentioned policies of the time) was to start a war and win it.

One additional factor: this was all almost prevented. Under the Bismarkian system of alliances, Germany was holding the continent of Europe together with its alliances where it was allied with both Russia and Austro-Hungary. If the Arch-Duke had been shot at this time, no war would have happened because due to alliances Russia would be allied with both Serbia and Germany, and therefore Austria-Hungary as well. Basically what would have happened was that Princip would have shot the arch-Duke and then Austria would have declared war on Serbia. Based on how the first days of the war went, Serbia would have won, and everything would have been over. Neither Germany nor Russia would have been able to act because they would be compromised by their alliances. However we all know that when Wilhelm II took over Germany he let Germany's alliance with Russia lapse, thus allowing the war to take place.

It would be rather ignorant to blame Serbia for World War One considering the circumstances of everything going on. Frankly, people were being terribly oppressed under the Austro-Hungarian empire. He should have known better than to be parading around Sarajevo (the current capital of Bosnia && where he was shot) when he knew the plight of the people living there, a plight he was partially responsible for. I think that every oppressed people have the right to fight for self-determination if they have to, so you could just as easily say it was the fault of Austria-Hungary for oppressing its minorities.