The Austro-Hungarian empire was a constitutional union of the Austrian Empire and the Kingdom of Hungary that existed from 1867 to 1918. The Empire was ruled by two monarchs, the Austrian monarchy and the Hungarian monarchy, with a single autonomous region called the Kingdom of Croatia-Slavonia. It was ruled by the royal House of Habsburg.
They were part of the "Triple Alliance" or "Central Powers", made principally of the Austro-Hungarian Empire, the German Empire, and the Ottoman Empire, with additional member Bulgaria. They were in opposition against the "Triple Entente" or "Allied Powers", made principally of the Russian Empire, France, the United Kingdom, the Kingdom of Italy, the Empire of Japan, and the United States of America.