John III Ducas Vatatzes

John III Ducas Vatatzes (1193-1254), Byzantine emperor (1222-1254), who, by his military successes and external policies, isolated the Latin Empire (see Byzantine Empire) and laid the groundwork for the reconquest of Constantinople (present-day İstanbul). Chosen to succeed his father-in-law, Theodore I Lascaris, John initially ruled only the area of Nicaea in Asia Minor, the remnant of the Byzantine Empire left him, but he expanded his realm by recovering the Latin conquests in Anatolia and winning new territories in Bulgaria and Epirus. A strong administrator, John did much to improve the social, cultural, and economic conditions of his subjects.