 Complete the following phrase:  As a result of heavy losses inflicted by the Ottomans in the Battle of Maritsa in 1371, the Serbian Empire had dissolved into several principalities. In the Battle of Kosovo in 1389, Serbian forces were again annihilated. Throughout the 15th and 16th centuries, constant struggles took place between various Serbian kingdoms and the Ottoman Empire. The turning point was the fall of Constantinople to the Turks. In 1459, following the siege, the temporary Serbian capital of Smederevo fell. Zeta was overrun by 1499. Belgrade was the last major Balkan city to endure Ottoman forces. Serbs, Hungarians, and European crusaders defeated the Turkish army in the 