Complex sentence: U.S. Army Major John C. Frémont arrived in California claiming to be on a mission to find a route to the Pacific (his mission officially was to find the source of the Arkansas River), he began encouraging a parallel rebellion among the Anglo-American settlers.
Split into simpler sentences: U.S. Army Major John C. Frémont arrived in California claiming to be on a mission to find a route to the Pacific (his mission officially was to find the source of the Arkansas River). He began encouraging a parallel rebellion among the Anglo-American settlers.