Complex sentence: Calhoun recommended him to the Michigan Territorial Governor, Lewis Cass, for a position on an expedition led by Cass to explore the wilderness region of Lake Superior and the lands west to the Mississippi River.
Split into simpler sentences: Calhoun recommended him to the Michigan Territorial Governor, Lewis Cass, for a position on an expedition. Cass was to explore the wilderness region of Lake Superior and the lands west to the Mississippi River.