John Pope was appointed a brigadier General of Volunteers on June 14, 1861 and ordered to Illinois on recruiting duty. Shortly thereafter, he was given command of the District of North and Central Missouri, north of the Missouri River and west of the Mississippi. Pope forced Confederate units out of north-central Missouri, taking over 800 prisoners at the small battle of Blackwater, in Johnson County, on December 18.
This action impressed Halleck who selected Pope to command the Army of the Mississippi with 25,000 men on February 23, 1862 and ordered down the Mississippi. A surprise march captured New Madrid on March 14 and then Island No. 10 on April 7. These actions gave the Union access on the Mississippi all the way to Memphis.
Promoted to Major General, Pope commanded under Halleck in northern Mississippi and was then given command of the Army of Virginia. He is best known as the commander at the worst Union defeat in the east at 2nd Manassas. He was then transferred to Minnesota for the rest of the war.
Side fact: Island No. 10 is so-named as it is the 10th major island in the Mississippi south of it's confluence with the Ohio, just south of Island No. 9 and north of Island No. 11 ;)