A papal legate (identified as Pelagius of Albano) came to Bulgaria in the summer of 1213. He continued his journey towards Constantinople, implying that his mediation contributed to the subsequent reconciliation between Boril and Henry. Boril desired peace because he had already realised that he would be unable to regain the Thracian territories lost to the Latin Empire; Henry wanted peace with Bulgaria in order to resume his war against Emperor Theodore I Laskaris. After lengthy negotiations, Henry married Boril's stepdaughter (whom modern historians wrongly call Maria) in late 1213 or early 1214.

In early 1214, Boril offered the hand of his unnamed daughter to Andrew II of Hungary's son and heir, Béla. Madgearu says he also renounced the lands that Andrew had claimed from Bulgaria (including Braničevo). In an attempt to conquer new lands, Boril launched an invasion of Serbia, laying siege to Niš in 1214, aided by troops sent by Henry. At the same time, Strez invaded Serbia from the south, although he was killed during his campaign. Boril was unable to seize Niš however, due to conflicts between the Bulgarian and Latin troops. Conflicts between Boril and the Latin troops prevented them from capturing the town.