It was November 1806. The explorers had gone without food for one day, then two. Their leader, not yet thirty, drove on, determined to ascend the great mountain. Waist deep in snow, he reluctantly turned back. But Zebulon Pike had not been defeated. His name remained on the unclimbed peak-and new adventures lay ahead of him and his republic.

In Citizen Explorer, historian Jared Orsi provides the first modern biography of this soldier and explorer, who rivaled contemporaries Meriwether Lewis and William Clark. Born in 1779, Pike joined the army and served in frontier posts in the...