In Battlefield 1, as with World War 1, there are a variety of factions (aka countries or "sides") which are involved in open combat. These factions often had changing, uneasy, or strange alliances, reflective of their geopolitical environments.
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.