Gregory IX's attempted expeditions to the Latin Empire of Constantinople: the crusade for the union of the Latin and Greek churches

Gregory IX's crusade (1236–1240) to safeguard the Latin Empire was the last expedition sponsored by the papacy before the fall of the Latin state in 1261. Like his predecessors, Innocent III and Honorius III, Gregory believed that an expedition against his fellow Christians was necessary to safeguard the land route to the Holy Land and to protect the Latin Empire itself. Gregory also shared with Innocent and Honorius the belief that this was a divinely appointed policy, symbolized by God giving the Greek Empire into Latin hands in retribution for Greek schismstic beliefs. But Gregory's policy had another facet to its justification. He accused the supporters of the Greeks, in particular John II Asen, king of Bulgaria, of sheltering heretics and of allowing a climate in which heresy could flourish. Gregory evolved a method of justifying war against the Greeks and their supporters analogous to that used elsewhere in Europe against those who sheltered heretics. He threatened the guilty with the loss of their lands under the provisions of the Fourth Lateran Council canon, Excommunicamus. To make the theoretical concrete, Gregory tried to form two expeditions, one composed of Europeans, the other of Hungarians and even Bulgarians, against the emperor of Nicaea. But the expeditions failed and Gregory's rationale for warring against the Greeks was not utilized by his immediate successors to the pontifical throne.