A successful soldier from a military family, John was chosen in about 1216 by Emperor Theodore I Laskaris as the second husband for his daughter Irene Laskarina and as heir to the throne, following the death of her first husband, Andronikos Palaiologos. This arrangement excluded members of the Laskarid family from the succession, and when John III Doukas Vatatzes became emperor in mid-December 1221, following Theodore I's death in November, he had to suppress opposition to his rule. The struggle ended with the Battle of Poimanenon in 1224, in which his opponents were defeated in spite of support from the Latin Empire of Constantinople. John III's victory led to territorial concessions by the Latin Empire in 1225, followed by John's incursion into Europe, where he seized Adrianople.