Complex sentence: In the 9th century CE, Byzantium established the Cherson theme to fend against incursions by the Rus'Khaganate, and the Crimean peninsula from this time was contested between Byzantium, Rus' and Khazaria.
Split into simpler sentences: In the 9th century CE, Byzantium established the Cherson theme to defend against incursions by the Rus'Khaganate. The Crimean peninsula from this time was contested between Byzantium, Rus' and Khazaria.