Complex sentence: In spring 1466, Sultan Mehmed marched with a large army against the Albanians and their leader, Skenderbeg, who had long resisted the Ottomans, and had repeatedly sought assistance from Italy.
Split into simpler sentences: In spring 1466, Sultan Mehmed marched with a large army against the Albanians. Under their leader, Skenderbeg, they had long resisted the Ottomans, and had repeatedly sought assistance from Italy.