Alliances contributed to the outbreak of World War I by forcing the great powers of Europe to go to war when their allies did. The two great alliances prior to the outbreak of war were the Central Powers, which consisted of Germany and Austria-Hungary, and the Entente, or Allied Powers, which consisted of Great Britain, Russia and France.
The spark that lit the fuse of war was the assassination of Franz Ferdinand, the Archduke of Austria, in Sarajevo on June 28, 1914. The assassin was a Serbian nationalist who wanted Austria-Hungary out of the Balkans. Austria-Hungary sent the Serbian government an ultimatum that promised war if specific demands were not met. However, Russia had promised to protect the Serbs and threatened retaliation. Once Austria invaded Serbia on July 28, 1914, the German army began mobilizing for war, prompting the Russian army to do likewise. Assuming that the Austro-Hungarians would take care of the Russians, Germany declared war on France on August 3, launching an invasion through Belgium, which was a neutral country. Due to its alliance with Belgium, Great Britain joined the war against the Germans on August 7. Because of the complex alliances necessitated by the balance of power theory of European relations, the outbreak of war in a small corner of the continent flared up into a continent-wide conflagration.