Croatia was a kingdom on the Balkans 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.
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, and Transylvania 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 Adriatic.