The Kingdom of Croatia-Slavonia (Croatian: Kraljevina Hrvatska i Slavonija; Hungarian: Horvát-Szlavónország or Horvát–Szlavón Királyság; German: Königreich Kroatien und Slawonien) was a nominally autonomous kingdom and constitutionally defined separate political nation[9][10] within the Austro-Hungarian Empire.

It was associated with the Kingdom of Hungary within the dual Austro-Hungarian state, being within the Lands of the Crown of St. Stephen, also known as Transleithania.

On 21 October 1918, Emperor Karl I, known as King Karlo IV in Croatia, issued a Trialist manifest, which was ratified by the Hungarian side on the next day and which unified all Croatian Crown Lands.

The claim was, for most of the time, supported by the Hungarian government, which backed Croatia–Slavonia in an effort to increase its share of the dual state.

The Croatian parliament, elected in a questionable manner, confirmed the subordination of Croatia–Slavonia to Hungary in 1868 with signing of Hungarian–Croatian union constitution called the Nagodba (Croatian–Hungarian Settlement, known also as Croatian–Hungarian Agreement or Hungarian–Croatian Compromise of 1868).

After the Austro-Hungarian Compromise of 1867 the only remaining open question of the new state was the status of Croatia, which would be solved with the Hungarian–Croatian Compromise of 1868 when agreement was reached between the Diet of Hungary on one hand and the Croatian Parliament on the other hand, with regard to the composition by a joint enactment of the constitutional questions at issue between them.

Both versions received Royal sanction and both as such became fundamental laws of the state with constitutional importance, pursuant to article 69. and 70. of the Settlement.

[29] Similarly to these affairs, trade matters including hawking, likewise with regard to societies which do not exist for public gain, and also with regard to passports, frontier police, citizenship and naturalization, the legislation was joint, but the executive in respect of these affairs was reserved to Kingdom of Croatia–Slavonia.

The Croatian Ban would now be nominated by the joint Croatian–Hungarian government led by the Hungarian Prime Minister, and appointed by the king.

Areas of "common" concern to Hungarians and Croats included finance, currency matters, commercial policy, the post office, and the railroad.

A draft law (bill), approved by the Diet, became a statute (an act) after the royal assent (sanction).

The King had the power to veto all legislation passed by the Diet and also to dissolve it and call new elections.

It was unicameral, but alongside 88 elected deputies (in 1888), 44 ex officio members were Croatian and Slavonian high nobility (male princes, counts and barons – similar to hereditary peers – over the age of 24 who paid at least 1,000 florins a year land tax), high dignitaries of the Roman Catholic, Greek Catholic and Eastern Orthodox churches and supreme county prefects (veliki župani) of all Croatian–Slavonian counties.

[38] Until 1914 it possessed three departments: At the head of the Autonomous Government in Croatia–Slavonia stood the Ban, who was responsible to the Croatian–Slavonian Diet.

[42] The counties were subsequently divided into a total of 77 districts (Croatian: kotari, similar to Austrian Bezirke) as governmental units.

18.307 of 16 November 1867 of the Department of the Interior of the Royal Country Government: The red–white–blue tricolor is the civil flag in the Kingdoms of Croatia and Slavonia, which with the united coat of arms of Croatia, Slavonia and Dalmatia with the crown of St. Stephen on top is the official flag for usage in autonomous affairs.

[47] It was also stated that the emblem for "joint affairs of the territories of the Hungarian Crown" is formed by the united coat of arms of Hungary and Dalmatia, Croatia and Slavonia.

The unofficial coat of arms was the preferred design and its widespread use was the reason that the Ban issued a Decree on 21 November 1914, stating that it had become "a custom in the Kingdoms of Croatia and Slavonia to use flags that are not adequate either in state-juridical or in political sense" and which strengthened flag related laws.

Additionally, Croats made up 5 percent of members in the Austro-Hungarian Common Army, a higher proportion than the percentage of the general population of the empire they composed.

[53] Notable Croatians in the Austro-Hungarian Army included Field Marshal Svetozar Boroević, commander of the Imperial and Royal Aviation Troops Emil Uzelac, commander of the Austro-Hungarian Navy Maximilian Njegovan and Josip Broz Tito who later became Marshal and President of the Socialist Federal Republic of Yugoslavia.

The Yugoslav Academy of Sciences and Arts and Matica hrvatska were the main cultural institutions in the kingdom.