With the assassination of Archduke Franz Ferdinand of Austria the first World War broke out. Originally called "The Great War", it wasn't until the rise of Nazism that it became "World War 1".
Europe, along with America, split into the Central Powers and Axis Powers. The cause of this split was the ultimatum that Austria sent to Serbia after a Serbian citizen assassinated the Archduke. Alliances that had been formed decades before were invoked and two sides of the war formed quickly.
Having begun in 1914, the war wouldn't end until the fall of 1918 when the Axis Powers defeated the Central Powers.