This was described from the German and Austrian side as a "Bund", an association. Austria-Hungary precipitated World War 1 and was the first to go to war on July 28th, 1914 with its declaration against the Kingdom of Serbia. Between August 1st and 4th the German Empire, which had given Austria-Hungary a "blank check" in relation to Serbia, in rapid succession declared war against Russia, France and Belgium. For the first three months of the war Germany and Austria-Hungary constituted the Central Power belligerents. From August 1 through October 1914 the alliance was therefore known as "der Zweibund", the Association of Two. On November 10th they were joined by the Ottoman Empire (Turkey) which officially enjoined hostilities against Russia, Great Britain and France. The alliance became "der Dreibund", The Association of Three. The Central Powers reached what was to be their full strength on October 14, 1915 with Bulgaria's opportunistic entry into the war against Serbia as massed German and Austro-Hungarian armies attacked from the north. The alliance therefore became "der Vierbund", The Association of Four, which was to remain until the disintegration of late 1918 as each of the Centrals sought armistice and peace in turn. http://www.ww1-propaganda-cards.com/central_powers_introduction.html
