Confederate general Leonidas Polk subsequently invaded Columbus, Kentucky, which ended Kentucky's policy of neutrality and turned it against the Confederacy. Grant used river transport and Andrew Hull Foote's gunboats of the Western Flotilla, threatening the Confederacy's "Gibraltar of the West" in Columbus, Kentucky. Although rebuffed at Belmont, Grant cut off Columbus. Confederate forces, lacking their gunboats, were forced to retreat and the Union took control of west Kentucky and opened Tennessee in March 1862.