World War I was started by the Austro-Hungarian Empire over the assassination of Archduke Ferdinand by a Serb nationalist in Sarajevo. The event gave Germany an opportunity to declare war on August 2, 1914. Two days later it invaded Belgium and France, bringing the British Empire, tsarist Russia, Italy, and later the United States into the conflict. These allies and Japan, Greece, Poland, Romania, and Serbia were called the Entente Powers, but France, the British Empire, Italy, Japan, and the United States referred to themselves as the Great Powers. Germany's allies, called the Central Powers, were the Austro-Hungarian Empire, the Turkish Ottoman Empire, and Bulgaria. The conflict was, until then, the bloodiest in history, resulting in more than 21 million casualties, with 8.5 million dead in slightly more than four years. During the conflict chemical weapons were used for the first time, and as a result of the harm they caused, their...