Britain entered World War I in 1914 to honor a treaty that promised to defend Belgium and to fulfill an alliance with France and Russia. The Germans attacked Liege, Belgium, in the first battle of World War I, and then marched on to attack France.
The scope of World War I had much to do with European treaties and alliances. The war was sparked when Serbian terrorists assassinated Archduke Franz Ferdinand, heir to the Austria-Hungary throne. Austria-Hungary retaliated by declaring war on Serbia, and Serbian ally Russia joined the fight. Germany allied with Austria-Hungary, and Britain joined the war just days after Germany entered Belgium.