Complex sentence: The Montenegrin army had laid siege on Shkodër, while the Serbians had conquered Kosovo and were advancing fast in north Albania, committing on the way many massacres and lootings of the Albanian settlements.
Split into simpler sentences: The Montenegrin army had laid siege on Shkodër, while the Serbians had conquered Kosovo and were advancing fast in north Albania. While conquering territory, their armies committed many massacres and lootings of the Albanian settlements.