Complex sentence: In 1231, when John of Brienne arrived in Constantinople, Ivan Asen II allied with the Nicaean Empire against the Latins and after the Nicaeans recognized the Bulgarian Patriarchate in 1235, he broke the union with the Papacy.
Split into simpler sentences: In 1231, when John of Brienne arrived in Constantinople, Ivan Asen II allied with the Nicaean Empire against the Latins. After the Nicaeans recognized the Bulgarian Patriarchate in 1235, Ivan Asen II broke his union with the Papacy.