 They maintained their rapid fire cannonade until the Union steam warships were safely past the southern forts and moving on New Orleans to begin the Union conquest of the lower Mississippi valley, which ultimately bisected the Confederacy and foredoomed its collapse.After New Orleans and the southern forts fell, Porter and his flotilla retired down the Mississippi and proceeded via Ship Island to Pensacola, Florida.However, they soon returned to the Mississippi to support Farragut's expedition up the river and reached sight of Vicksburg, Mississippi, 20 June.