The main actors at the war's outbreak were enmeshed in the two major alliance systems of the early twentieth century. The Triple Alliance consisted of Germany, Austria-Hungary, and Italy; and the Triple Entente was made up of Great Britain, France, and Russia. Many of the nations also had alliance obligations to smaller nations, as Russia did with Serbia and England did, more or less, with Belgium. When the war broke out, of course, Italy did not join Austria-Hungary and Germany, but the Ottoman Empire, the other major player in the war, did. By the end of the war, the United States and many other nations around the world had joined the Allied fight against Germany, Austria-Hungary, the Ottoman Empire and Bulgaria, who rounded out the so-called Central Powers.