Complex sentence: During his administration, De Rosas had confrontations with the Franciscans (mainly because of De Rosas' dealing with the Native Americans, whom he forced to work as slaves), who promoted a revolt of the citizens of New Mexico against him.
Split into simpler sentences: During his administration, De Rosas had confrontations with the Franciscans, mainly because of De Rosas' dealing with the Native Americans, whom he forced to work as slaves. The Franciscans promoted a revolt of the citizens of New Mexico against him.