Complex sentence: Alexius sent Tatikios back with naval assistance from Manuel Boutoumites, but although he was able to defeat Abu'l Qasim, the governor of the city, in Bithynia, he could not recapture the city.
Split into simpler sentences: Alexius sent Tatikios back with naval assistance from Manuel Boutoumites. Although Tatikios was able to defeat Abu'l Qasim, in Bithynia, he could not recapture the city.