The Byzantine historian Niketas Choniates mentioned that Theodor-Peter designated Kaloyan "to assist him in his labors and share in his rule" at an unspecified time. Kaloyan became the sole ruler of Bulgaria after Theodor-Peter was murdered in 1197. Shortly afterwards he attacked the Byzantine province of Thrace and launched frequent raids against it during the following months. Around this time, he sent a letter to Pope Innocent III, urging him to dispatch an envoy to Bulgaria. He wanted to persuade the pope to acknowledge his rule in Bulgaria. Innocent eagerly entered into correspondence with Kaloyan because the reunification of the Christian denominations under his authority was one of his principal objectives.