The outbreak of World War I was triggered by the assassination of Archduke Franz Ferdinand of Austria-Hungary by a Serbian nationalist on June 28, 1914. This event led to a series of diplomatic crises between Austria-Hungary and Serbia, which eventually led to a declaration of war by Austria-Hungary on Serbia on July 28, 1914. This declaration of war was quickly followed by declarations of war by Germany on Russia and France, and by Britain on Germany. Austria-Hungary was allied with Germany, and Russia was allied with France and Britain. Belgium was a neutral country, but was invaded by Germany on August 4, 1914, which led to Britain declaring war on Germany.
