WWI: World War I was fought between the Allied Powers ( France, Russia, and Britain, the United States also fought on the side of the Allies after 1917.) and the Central Powers. Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria. Although there were a number of causes for the war, the assassination of Austrian Archduke Franz Ferdinand was the main reason for starting the war. After the assassination, Austria declared war on Serbia. Then Russia and other European countries joined the war to protect their allies.