Why did Austria and Hungary unite?

They united in 1526, directly after and because of the Battle of Mohacs. (from the rather cheerfully named Executed Today) In 1526, the Ottoman Empire was advancing through Europe. read more

Austria-Hungary was a multinational state and one of the world's great powers at the time. Austria-Hungary was geographically the second-largest country in Europe after the Russian Empire, at 621,538 km 2 (239,977 sq mi), and the third-most populous (after Russia and the German Empire). read more

This long chain reaction eventually led to Russia, France, the United Kingdom, many other European nations, and eventually the United States, known as the Allies, fighting against Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria, known as the Central Powers. read more