He deposed John IV Lascaris, whose regent he had been. He capitalized on the attempts of his predecessors in the Empire of Nicaea to restore the throne in Constantinople and put the Empire back on the map as a force to be reckoned with. He realized that the danger existed that the Latin West, particularly his neighbors in Italy (Charles of Anjou, Pope Martin IV, and the Venetians) would be unified against him and set out to avoid the mistakes of Manuel I.
To drive a wedge between the pope and the others he decided to unify the Byzantine church with the Catholic one; a tenuous union between the Greek and Latin church was signed at the Council of Lyons in 1274. He did so at a great price at home: his prisons filled with many disgruntled people of Orthodox faith. For a while the wedge worked but in the end Martin IV (working in part for Charles of Anjou) excommunicated him. Then he needed a new wedge and used truly "Byzantine" diplomacy to get the Catalans of Peter III of Aragon to attack Sicily, thus cutting Charles's kingdom in half. (See the Sicilian Vespers massacre.)
John IV (Empire of Nicaea)
Baldwin II (Latin Empire)