Michael VIII Palaiologos's reign saw considerable recovery of Byzantine power, including the enlargement of the Byzantine army and navy. It would also include the reconstruction of the city of Constantinople, and the increase of its population. He reestablished the University of Constantinople, which led to what is regarded as the Palaiologan Renaissance between the 13th and 15th centuries. It was also at this time that the focus of the Byzantine military shifted to the Balkans, against the Bulgarians, leaving the Anatolian frontier neglected. His successors could not compensate for this change of focus, and both the Arsenite schism and two civil wars (the Byzantine civil war of 1321–1328, and the Byzantine civil war of 1341–1347) undermined further efforts toward territorial consolidation and recovery, draining the empire's strength, economy, and resources. Regular conflict between Byzantine successor states such as the Empire of Thessalonica, Trebizond, Epirus and Serbia resulted in permanent fragmentation of former Byzantine territory and opportunity for increasingly successful conquests of expansive territories by post-Seljuk Anatolian beyliks, most notably that of Osman, later called the Ottoman Empire.