World War I was truly one of the most tragic events of the twentieth century. The war began over a terrorist act in the provinces of the fading Austro-Hungarian Empire and could have been avoided if Germany, Russia, and France hadn't felt compelled to obey secret treaties they had signed years before. Those secret treaties turned a small conflict into one that involved every major country in Europe and eventually many other nations from around the world. In the course of just over four years of war, nearly ten million soldiers and civilians lost their lives; billions of dollars were spent on killing machines-guns, tanks, submarines-and the economies of most of the warring countries were severely disrupted; two great empires-the Austro-Hungarian Empire and the Ottoman Empire-collapsed in defeat.