From Alternative History

Croatia was a kingdom on the Balcans which was traditionally under the crown of Hungary.

After the Rum-Seljuks had a bit recovered from the Black Death, they stroke against Hungary again (1396-1400). They occupied Moldavia, Bosnia and Slavonia (East Croatia). During 1480-1492, they broke Hungary up, leaving only Slovakia and the western third to the king. The center and Croatia were annexed, Transsylvania became a tributary.

After the Second French-Seljuk War 1635-46, in the peace of Györ, Bourbon Hungary acquired Croatia and Slavonia (and Transsylvania and the Banat).

When Russia diplomatically clashed with Hungary in 1834, emperor Alessandro I feared Russia invading Hungary at the wrong time. So he suggested the Hungarian Division instead: Austria went to Germany, Croatia to New Rome, Transsylvania became a Russian protectorate. New Rome also acquires most of OTL Slovenia (which was once Austrian, until Austria was inherited by Hungary in 1816), so Germany was cut off from the Adria.

During the anti-Russian War, in August 1850, the New Romans defeated the attacking Serbian-Romanian-Bulgarian troops at the battle of Vinkovci, Croatia.