Complex sentence: Roughly contemporaneous with the Lewis and Clark Expedition, the military expedition was led by United States Army Captain Zebulon Pike and was the first official American effort to explore the western Great Plains and the Rocky Mountains in Colorado.
Split into simpler sentences: Roughly contemporaneous with the Lewis and Clark Expedition, the military expedition was led by United States Army Captain Zebulon Pike. It was the first official American effort to explore the western Great Plains and the Rocky Mountains in Colorado.