One day when Archduke Franz Ferdinand was visiting Sarajevo in Serbia, a bunch of bunk ass terrorists decided to assassinate him. This caused many events to happen; Austria- Hungary wanted Bosnia back from Serbia, they said no so Austria- Hungary declared war on serbia. Russia had a pact with serbia so they declared war on Austria- Hungary. Germany had a pact with Austria- Hungary so they declared war on Russia. finally Brittain and France had pacts with Russia so they declared war on Germany.