News about John of Brienne's election to the regency in the Latin Empire outraged Ivan Asen. He sent envoys to the Ecumenical Patriarch Germanus II to Nicaea to start negotiations about the position of the Bulgarian Church. Pope Gregory IX urged Andrew II of Hungary to launch a crusade against the enemies of the Latin Empire on 9 May 1231, most probably in reference to Ivan Asen's hostile actions, according to Madgearu. Béla IV of Hungary invaded Bulgaria and captured Belgrade and Braničevo in late 1231 or in 1232, but the Bulgarians reconquered the lost territories already in the early 1230s. The Hungarians seized the Bulgarian fortress at Severin (now Drobeta-Turnu Severin in Romania) to the north of the Lower Danube and established a border province, known as the Banate of Szörény, to prevent the Bulgarians from expanding to the north.