During June of 1914 the Archduke of the Austrian-Hungarian Empire was assassinated by a young Bosnian Serb with a pistol . Austria-Hungary issued an ultimatum to Serbia . Russia , Serbia’s ally , mobilized her armies . Germany , Austria-Hungary’s ally , invaded neutral Belgium in order to implement a preconceived plan to attack France . Britain , France’s ally , jumped in . Japan , an ally of Britain, declared war . Allies honored alliances and soon a global war had begun .
By 1918 , when the war ended with an armistice , more than nine million combatants had been killed .
Americans called it the European War . President Wilson promised that the U.S. would stay out of it .
2014 is the 100th anniversary of World War One .