Complex sentence: Sympathy for Selim persisted, however, and in 1808 an army under Mustafa Bayrakdar set out for Istanbul in order to bring Selim back to the throne, Mustafa ordered the execution of Selim as well as another brother, Mahmud.
Split into simpler sentences: Sympathy for Selim persisted, however, and in 1808 an army under Mustafa Bayrakdar set out for Istanbul in order to bring Selim back to the throne. In response, Mustafa ordered the execution of Selim as well as another brother, Mahmud.