The fall of New Orleans in April 1862 capped the beginning of an 18-month drive to control Vicksburg and the Mississippi River. Vicksburg, high atop bluffs and protected by artillery and a maze of bayous, held a strategic position, while Confederate river fortifications interrupted the flow of Northern troops, supplies, and commerce. Driving southward from Tennessee and northward from the Gulf, Federal troops eventually forced the surrender of Vicksburg, opening the Mississippi, dealing a psychological blow to the Confederacy, and splitting the South in two so that men and munitions could no longer move across the river. The campaign intensified when Ulysses S. Grant took command of the Department of the Tennessee and Lt. Gen. John C. Pemberton was given charge of 50,000 Confederate troops defending the Mississippi. Ordered to clear the river of Confederate resistance, Grant planned to draw defenders north while a column under Maj. Gen. William T. Sherman moved south from Memphis. Grant led 40,000 troops from La Grange, Tennessee, toward Grenada, Mississippi, but fierce resistance along the Yalobusha riverbank thwarted the plan, and Confederate cavalry under Maj. Gen. Earl Van Dorn captured the Federal supply base at Holly Springs, while Brig. Gen. Nathan Bedford Forrest's raiders struck the Mobile & Ohio Railroad, forcing Grant to retreat to Memphis. Confederate troops then decisively beat Sherman's forces at Chickasaw Bayou. Grant next moved 50 miles down the west side of the river from Milliken's Bend, Louisiana, to Hard Times to disrupt Southern communications, confuse the Confederates, and divert Pemberton's troops. At the same time, Union Col. Benjamin H. Grierson led 1,700 cavalry from La Grange through Mississippi to Baton Rouge, destroying rolling stock, bridges, trestles, track, and telegraph lines during a 16-day, 475-mile ride and drawing off Pemberton's cavalry and one infantry division. In southern Louisiana, Adm. David D. Porter maneuvered gunboats to secure the river at Grand Gulf, forcing Grant farther downriver to cross at Bruinsburg. Grant then fought his way toward Vicksburg on the eastern side of the river, overwhelming and scattering Confederate troops, and began the final siege that ended with Confederate surrender on July 4, 1863. Five days later Port Hudson fell, and domination of the Mississippi River was complete.