Complex sentence: In 1880 Ulcinj was occupied by Great Power forces and has been given to Principality of Montenegro, after the WWI Montenegro was part of Kingdom of Serbs, Croats and Slovenes.
Split into simpler sentences: In 1880 Ulcinj was occupied by Great Power forces and had been given to Principality of Montenegro. After World War I Montenegro was part of the Kingdom of Serbs, Croats and Slovenes.