The Central Powers in World War I consisted of Germany, Austria-Hungary, Bulgaria and the Ottoman Empire. They were so-called because the four countries lay between Russia in the east and the United Kingdom and France in the west. The Central Powers were also called the Quadruple Alliance, although Bulgaria and the Ottoman Empire did not join until after the beginning of World War I.
When the assassination of Archduke Franz Ferdinand in July 1914 incited war between Austria-Hungary and Serbia, Russia threatened to defend Serbia against any invasion. Germany publicly declared its support for Austria-Hungary and stated it would enter the war to protect its ally if necessary. Germany formally declared war against Russia in August. The Ottomans joined the alliance in the same month, although the empire did not formally enter the war until November, when Germany demanded its support. Bulgaria was drawn into the Powers by its alliance with the Ottomans, and became part of the Quadruple Alliance in October of 1915.
The countries of the Central Powers signed treaties to end the war independent of one another. Bulgaria was the first to lay down arms in September of 1918, followed by the Ottoman Empire the following month. Austria and Hungary, having collapsed into two separate countries, capitulated in the first week of November, with Germany signing the armistice on Nov. 11, 1918.