The assassination of Archduke Franz Ferdinand led Austria to issue an ultimatum to Serbia, threatening war. As alliances formed over the previous decades took hold, Russia mobilized to support Serbia, Germany in turn mobilized to support Austria, and France supported Russia. Germany then invaded Belgium, drawing Britain into the war on August 4th, 1914, despite none of the nations truly expecting or wanting a prolonged continental war. Tensions had been rising as the balance of power shifted in Europe.