Halleck appointed Pope to command the Army of the Mississippi (and the District of the Mississippi, Department of the Missouri) on February 23, 1862. Given 25,000 men, he was ordered to clear Confederate obstacles on the Mississippi River. He made a surprise march on New Madrid, Missouri, and captured it on March 14. He then orchestrated a campaign to capture Island No. 10, a strongly fortified post garrisoned by 12,000 men and 58 guns. Pope's engineers cut a channel that allowed him to bypass the island. Assisted by the gunboats of Captain Andrew H. Foote, he landed his men on the opposite shore, which isolated the defenders. The island garrison surrendered on April 7, 1862, freeing Union navigation of the Mississippi as far south as Memphis.