Complex sentence: In 1881, Prince Carol I was crowned as "King of the Romanians" (King of Romania), that had been recognized in 1878 in written documents by the Porte and the Sultan as an Independent Principality, and thus no longer an Ottoman vassal.
Split into simpler sentences: In 1881, Prince Carol I was crowned as "King of the Romanians" (King of Romania). Romania had been recognized in 1878 in written documents by the Porte and the Sultan as an Independent Principality, and thus no longer an Ottoman vassal.