Pope was a graduate of the United States Military Academy in 1842. He served in the Mexican–American War and had numerous assignments as a topographical engineer and surveyor in Florida, New Mexico, and Minnesota. He spent much of the last decade before the Civil War surveying possible southern routes for the proposed First Transcontinental Railroad. He was an early appointee as a Union brigadier general of volunteers and served initially under Maj. Gen. John C. Frémont. He achieved initial success against Brig. Gen. Sterling Price in Missouri, then led a successful campaign that captured Island No. 10 on the Mississippi River. This inspired the Lincoln administration to bring him to the Eastern Theater to lead the newly formed Army of Virginia.