Henry forces Theodore to sign a treaty at Nymphaeum in 1214, defining the borders of their two realms and ceding the northwestern portions of Asia Minor to Henry; he also makes an alliance through marriage with the Bulgarian tsar Boril.

Thus, through diplomacy Henry is able to ensure the security of the Latin empire.

Ivan Asen II is a son of Ivan Asen I, one of the two founders of the Asen dynasty and the Second Bulgarian Empire, and Elena.

After the death of his uncle Kaloyan in 1207, Ivan Asen's cousin, Boril, had usurped the throne and forced him to flee to the Rus principality of Galicia-Volhynia.

Bulgaria’s alliance with the Latin Empire, the Kingdom of Hungary, and the Despotate of Epirus had dragged Boril into a war against Serbia, in which Boril has made little headway, especially after the murder of his brother Strez in 1215.

With the death of Henry in 1216 and the departure of Andrew II on the Fifth Crusade, Boril had been left essentially without strong supporters.

In 1217 or 1218 Ivan Asen, Boril's cousin, returns from exile with the support of Galicia-Volhynia and defeats Boril, who locks himself up in Tărnovo.

After a siege of perhaps seven months (rather than the "seven years" of the Byzantine sources), Boril flees the capital, which surrenders to Ivan Asen.

Boril is captured during his escape, and is blinded and relegated to a monastery.

Having established himself on the throne, Ivan Asen II sets about recovering the losses sustained by Bulgaria during the reign of Boril.

A good soldier and administrator, Ivan restores law and order, controlling the boyars.