Political map of Europe & the Mediterranean 4 August 1914 (Outbreak of the Great War): On July 28, Austria-Hungary attacked Serbia (Serbian Campaign (World War I)), starting the First World War (World War I). Facing war with France and Russia (Franco-Russian Alliance), Germany attempted to quickly defeat France first by moving through neutral Belgium (Schlieffen Plan). However, the violation of Belgium's neutrality prompted Britain to join the war on the side of the French.