Treaty of Viterbo

The Treaty of Viterbo (or the Treaties of Viterbo) was a pair of agreements made by Charles I of Sicily with Baldwin II of Constantinople and William II Villehardouin, Prince of Achaea, on 24 and 27 May 1267, which transferred much of the rights to the defunct Latin Empire from Baldwin to Charles.

The recapture of Constantinople by the Greeks in 1261 had sent Baldwin, the Latin Emperor of Constantinople, always impoverished, to seek aid in Western Europe for a crusade to recapture the throne of the Latin Empire for him. For some time, he had expected aid from Manfred of Sicily, who hoped (like his father, Emperor Frederick II) that a crusade might put him in better standing with the Papacy; but Manfred was ousted from Sicily in 1266 by Charles, acting for Pope Clement IV, severely discomfiting Baldwin. Baldwin was residing at the Papal court in Viterbo in 1267 when Clement arranged to reconcile the Emperor and the newly seated King. As the principal vassal of what remained of Baldwin's empire, William was keen to gain an overlord who might be the giver rather than the recipient of subsidy, and who could help defend the Principality against the Despotate of Epirus, which had worsted him in 1259 at the Battle of Pelagonia. His chancellor, Leonardo of Veruli, was sent to Viterbo to negotiate the treaty.

The War of the Euboeote Succession (an inheritance dispute between William and the Lombard lords, or "terciers", of Euboea, backed by Venice) and his defeat at Pelagonia had left William's principality exhausted in resources. To obtain the aid of Charles, he was willing to cede Charles the Principality in return for life tenure there. The proposed marriage of William's elder daughter Isabelle with the Byzantine emperor's son Andronikos was to be broken off, and she was to marry Charles' son Philip. Furthermore, Charles would have the reversion of the principality should the couple have no issue, disinheriting Isabelle. William, somewhat reluctantly, agreed to these terms.

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