When did World War I begin?

World War I began on July 28, 1914.
The immediate cause of the war was the assassination of Archduke Franz Ferdinand of Austria by a Serbian nationalist who wanted to free his nation from Austro-Hungarian rule. Almost all the countries of Europe were involved in alliances, and when Austria-Hungary and Serbia faced off, most of the other European nations were dragged into the conflict.
The causes of the war, however, went much deeper. The groundwork for the conflict began with the unification of Germany in 1870 and its defeat of France in the Franco-Prussian war. The French were looking to even the score from that conflict, while Germany was moving to rival Great Britain as a world power. Germany threatened to challenge British control of the seas, something the British were not about to allow. Therefore, with the death of Archduke Ferdinand in 1914, tensions that had long been simmering broke into war.