WWI was not really caused by some Serb nationalist. Sheesh, you'd think he operated in a vacuum. The war was going to start regardless, any old spark would have done the trick. The Austro-Hungarian empire was weak and needed something, anything to attempt to regain power. The Ottoman empire by that time was in a similar position. Russia under the Tsars was in ferment. The Germans hoped to make their push for hegemony in Europe. The French were dead set against that after the wars of the previous century. Britain was itching for fight too.