Complex sentence: The Cumans first encountered the Rus'in 1055, leading to a peace agreement; in 1061, however, the Cumans invaded and devastated the Rus' Pereiaslav principality.
Split into simpler sentences: The Cumans first encountered the Rus'in 1055, leading to a peace agreement. In 1061, however, the Cumans invaded and devastated the Rus' Pereiaslav principality.