The original Allied powers united because of a web of bilateral treaties activated in the wake of Austrian Archduke Franz Ferdinand’s assassination by a Serbian nationalist on June 28, 1914. In a domino-like chain reaction, Austria-Hungary declared war on Serbia (July 28); Serbia’s protector, Russia, mobilized its forces to defend Serbia (July 30); and Austria-Hungary’s ally, Germany, declared war against Russia (August 1), which also necessitated declaring war against France (August 3). Germany’s plan for fighting France required it to first invade Belgium (August 3–4). Great Britain was obligated to defend Belgium, so it responded by declaring war against Germany (August 4). And as an ally of Great Britain, Japan declared war against Germany on August 23.