Political map of Russia & the former Soviet Union 3 August 1914 (Outbreak of the Great War): The assassination of Archduke Franz Ferdinand of Austria-Hungary (Assassination of Archduke Franz Ferdinand of Austria) by a Bosnian Serb in June 1914 triggered a series of political crises (July Crisis) across Europe. On July 28, Austria-Hungary declared war on Serbia, a country protected by Russian guarantees. Russia responded by mobilizing its armies, which in turn led to war with Austria-Hungary's ally Germany. A few days later, France and then Britain joined the war (World War I) on the side of Russia.