World War I was fomenting due to a tangled web of alliances between the various European powers, but the catalyst for the war was the assassination of the Archduke of Austria, Franz Ferdinand. The assassination occurred on June 28, 1914, at the hands of Serbian nationalist Gavrilo Princip of the Black Hand. His subjects did not view the archduke favorably, but it served as an excuse for Austria to start a war with Serbia.
Austria felt that Serbia was overreaching its territorial boundaries and that the only way to forestall it was a preventive war. The death of Ferdinand provided a convenient excuse to go to war with Serbia. In support of Serbia, Russia mobilized its forces against Austria-Hungary, who had declared war on Serbia. Germany declared war on Russia. The United Kingdom and France, allies of Russia, also entered the conflict.
A web of alliances existed between many European countries. Austria allied with Germany, and Serbia allied with Russia. The alliances obligated each country to go to war on behalf of its allies. This dragged more and more countries into the war. The war pitted the Central Powers, consisting of Germany, Austria-Hungary, the Ottoman Empire and Bulgaria, against the Allied Powers, consisting of the United Kingdom, France, Russia and Italy. The United States ultimately entered the war on the Allied side.