Complex sentence: Alexander's subsequent fate is unknown until the end of the Ottoman Interregnum in 1413, when Mehmed I (r.1413--1421) had established himself as ruler of the Ottoman Empire and had retaken the region of Smyrna on the eastern Aegean coast.
Split into simpler sentences: Alexander's subsequent fate is unknown until the end of the Ottoman Interregnum in 1413. In that year, Mehmed I (r.1413--1421) had established himself as ruler of the Ottoman Empire and had retaken the region of Smyrna on the eastern Aegean coast.