Complex sentence: After being part of the Ottoman Empire from the 15th to the early 20th century, Kosovo again found itself within a Serbian state when it was incorporated into the Kingdom of Serbia as a result of Ottoman defeat in the First Balkan War (1912--13).
Split into simpler sentences: After being part of the Ottoman Empire from the 15th to the early 20th century. In the next century Kosovo again found itself within a Serbian state when it was incorporated into the Kingdom of Serbia as a result of Ottoman defeat in the First Balkan War (1912--13).