The Austro-Hungarian Empire or the Dual Monarchy, was a constitutional union of the Austria and the Kingdom of Hungary that existed from 1867 to 1918 A.D. The state had two capitals Vienna in Austria and Budapest in Hungary.
In 1914 A.D. "The Great War" began when Archduke Franz Ferdinand, heir to the thrones of the Austro-Hungarian Empire, was killed by an assassin from Serbia. Austria-Hungary declared war on Serbia. Due to various treaties, Germany allied with Austria-Hungary while Russia aided Serbia; in turn, France was pledged to defend Russia. The United Kingdom attempted to remain outside the conflict, but when Germany invaded Belgium to flank France's border defenses, the British joined the war to honor arrangements with Belgium.
In 1918 A.D., the central powers was defeated and signed an armistice.
Post World War I
The Austro-Hungarian Empire was broken up at the end of World War I. The new countries of Hungary and Austria were formed along with Czechoslovakia, Poland, and Yugoslavia. Transylvania was returned to Romania.
A Hungarian republic was proclaimed following a revolution; meanwhile, Karl I renounced participation in affairs of state and fled to Switzerland, making him the last Habsburg ruler. Austria would become a republic on 12th November 1918.