Complex sentence: By then, the Ottoman Turks had already gained a foothold in the Balkans; first defeating the Serbs at the Battle of Kosovo and expanding westward, the Turks eventually conquered all of Bosnia and portions of neighboring Croatia.
Split into simpler sentences: By then, the Ottoman Turks had already gained a foothold in the Balkans. First defeating the Serbs at the Battle of Kosovo and expanding westward, the Turks eventually conquered all of Bosnia and portions of neighboring Croatia.