FF was an Austrian archduke. Austria-Hungary found his assassination by Serbian nationalists unacceptable. But Russia mobilized to help its Balkan allies. Germany, due to its pact with Austria-Hungary, was forced to mobilize to help their allies. Knowing that France had a pact with Russia, Germany attacked it, heading through Belgium. Britain, since it remained an ally of Belgium, and to a lesser extent France, declared war on Germany. And so the Great War began.