Ivan Asen II was succeeded by his infant son Kaliman I. Despite the initial success against the Mongols, the regency of the new emperor decided to avoid further raids and chose to pay them tribute instead. The lack of a strong monarch and increasing rivalries among the nobility caused Bulgaria to rapidly decline. Its main rival Nicaea avoided Mongol raids and gained power in the Balkans. After the death of 12-year-old Kaliman I in 1246, the throne was succeeded by several short-reigned rulers. The weakness of the new government was exposed when the Nicaean army conquered large areas in southern Thrace, the Rhodopes, and Macedonia—including Adrianople, Tsepina, Stanimaka, Melnik, Serres, Skopje, and Ohrid—meeting little resistance. The Hungarians also exploited Bulgarian weakness, occupying Belgrade and Braničevo.