War broke out in the summer of 1914 because several of the nations of Europe had differences that could not be settled by peaceful means. The diplomats continued to do their work up to the moment that the first soldiers passed into the territory of the enemy. Their efforts were unavailing. There were deep and painful conflicts among the countries of Europe that led to the unprecedented calamity of a war that lasted four and a half years, that killed more Europeans than any event since the Black Death of the fourteenth century, and that altered the shape of European life and the geopolitical map. Princip's rash act, two shots fired without aiming by a lad of nineteen who had just learned to use a revolver, set the crisis in motion.