The Lands of the Holy Hungarian Crown (comprising the Kingdom of Hungary proper including Transylvania and the Kingdom of Croatia-Slavonia) were granted equal status with the rest of the Habsburg monarchy. Each of the two states comprising Austria-Hungary exercised considerable independence, with certain institutions, notably the reigning house, defence, foreign affairs, and finances for common expenditures, remaining under joint management. This arrangement lasted until 1918, when the Central Powers went down in defeat in World War I.
The new borders set in 1920 by the Treaty of Trianon ceded 72% of the historically Hungarian territory of the Kingdom of Hungary to the neighbouring states. The beneficiaries were Romania, the newly formed states of Czechoslovakia, and the Kingdom of Serbs, Croats and Slovenes
Later in 1920, a coalition of right-wing political forces united, and reinstated Hungary's status as a constitutional monarchy. Selection of the new King was delayed due to civil infighting, and a regent was appointed to represent the monarchy. Former Austro-Hungarian navy admiral Miklós Horthy became that regent.
In 1944, Nazi Germany occupied and abolished the Kingdom of Hungary and created a pro-nazi puppet regime headed by Ferenc Szalaszi.