Since 1237, however, Frederick II had entered into close relations with the emperor of Nicaea John III Vatatze, because the latter had acquired in those years a hegemonic position over the states of Epirus and Thessalonica, which culminated in 1242 in the conquest of Thessalonica.. Thus, already in 1238 John III sent in Italy a small contingent of troops, and in July of that year argued in a rather symbolic Svevo during the siege of Brescia, with Federico II, which in turn prevented the ‘crusade’ desired and organized by Pope Gregory IX for the garrison of Constantinople, besieged in that year by the emperor of Nicea. The culminating episode of this rapprochement between the two emperors was the marriage between Vatatze himself and Constance (the illegitimate daughter that Frederick had generated with Bianca Lancia ; see Constance / Anna, empress of Nicaea ), celebrated at the end of 1240 or at the beginning of 1241, and not already in 1244 as often stated by historiography. Constance assumed the Greek name Anna and the three cities of Achyra, Stylarion and Keramos, near Smyrna in Asia Minor, were assigned to her as a dowry by her husband.. However, the cooperation between the two emperors was short-lived, probably because Vatatze saw disappointed the hope of concrete military support from his father-in-law in view of the reconquest of Constantinople.
Historiography (from Festa, in The Greek Letters , 1894, up to Martin, 2002, with the sole exception of Kresten, 1991), on the other hand, has always spoken of a lasting alliance between the two sovereigns until the death of Frederick II in 1250, citing as proof three letters, written in Greek and addressed to his son-in-law in 1250, with which the Swabian emperor wished with grandiloquent expressions the understanding between the two Empires. But those letters (all three undated and handed down only as an appendix to a manuscript of the Laurentian Library of Florence containing the tragedies of Sophocles) were not at all drawn up in the Swabian chancellery. Undoubtedly, however, these are stylistic exercises, probably conceived in the context of the ‘Otrantine circle’ that flourished around Giovanni Grasso da Otranto and Giorgio da Gallipoli, with the aim of prefiguring the utopia of a close collaboration between the two emperors against the papacy. An illusion that did not withstand, however, the impact of historical reality.
On the other hand, a rapprochement between Frederick II and the mortal enemy of Giovanni Vatatze, namely the Latin emperor of Constantinople, Baldwin II of Courtenay, can be observed since 1244. In those years Baldwin stayed in the West in search of money and troops to defend Constantinople from the superior power of the emperor of Nicaea. In 1244 the Swabian negotiated a truce between the two eastern emperors, while Baldwin tried to reconcile Frederick with Pope Innocent IVand even the following year he defended the Swabian in the famous council of Lyon.
The revelation of the new friendship between Frederick II and Baldwin II is the attitude held by the Swabian in the struggle unleashed for the marquisate of Namur between the Latin emperor of Constantinople and Giovanni d’Avesnes, count of Annonia. In fact, Baldwin had committed his hereditary marquisate, which was also a fief of the Western Empire, for 50,000 lire of Paris to King Louis IX of France. without Federico’s authorization. However, while the Swabian continued to support Baldwin, the antiré of the Romans William of Holland, elected in 1246 on the initiative of Pope Innocent IV, in October 1247 seized the marquisate of Namur from Baldwin and made him fief of John of Avesnes. The consequence of the convergence between Frederick and Courtenay was the rapprochement between the pope and Giovanni Vatatze. As early as 1246 Innocent IV showed himself willing to sacrifice the Latin Empire of Constantinople to the Greeks, while in 1249 the emperor of Nicaea formally began negotiations for the union of the two Churches. Therefore the contacts between Vatatze and the Swabian were completely exhausted since 1247 and in summer 1250 relations between the two emperors had already deteriorated so much that in those days Frederick II blocked an embassy of John III in the Kingdom of Sicily, destined to reach the papal Curia in Lyon for the continuation of the negotiations for the union of the Churches. Probably only the death of Svevo prevented a complete break with his son-in-law. On the other hand, no source mentions relations of Frederick II with both the princes of Achaea and the emperors of Trebizond.
Ultimately we can identify three clearly distinct phases in the relations between Frederick II and the successor states of the Byzantine Empire: the first of the years 1229-1236, with an epilogue in 1240, was characterized by at least sporadic contacts with the emperors of Thessalonica Theodore and Manuel Angelo Duca, as well as with Michele II Angelo Duca, lord of Epirus since 1236. The second phase, between 1237 and 1244-1245, was instead marked by the close collaboration with the son-in-law Giovanni III Vatatze; while the third, from 1244-1245 to 1250, saw the alliance with the Latin emperor Baldwin II, an approach probably functional to demonstrate to the King of France, Louis IX, the sincere intention of the Swabian to protect the to Pope Innocent IV.