The basic issue that led to World War I was the assassination of the heir to the Habsburg throne, Archduke Francis Ferdinand, in 1914. However, this was only the pretext for the war, with further causes being the mutual defense alliances in Europe, militarism, nationalism and imperialism.
World War I began in the summer of 1914, when Austria-Hungary declared war on Serbia days after the assassination of the Archduke. Russia, which was an ally of Serbia, prepared for war, which prompted Germany to declare war on Russia. The conflict escalated and eventually included all countries involved in mutual defense alliances.