Union General William T. Sherman once called the Mississippi River "the spinal column of America." Certainly the goods and supplies that traveled on the river were the backbone of Confederate strength; the town of Vicksburg, Mississippi, was vital to control of the river. In December 1862, Sherman's land forces, intending to halt the shipment of war supplies to the Confederacy, assaulted the city. It was an effort doomed to fail: the town was well fortified and its hills and sheer cliffs proved formidable. Sherman then retreated south, joining with the forces of General Ulysses S. Grant. Their combined troops, hoping to let the Union navy circumvent Vicksburg, tried to cut levees and canals from the Yazoo River and bayous. These attempts also failed.

On the night of April 16, 1863, a large part of the Union fleet, under the command of Rear Admiral David D. Porter, dared to pass directly before Vicksburg's heavy artillery. Seven ironclads sustained minor damage; one transport sank in flames. Once downstream, the surviving ships shuttled Union forces across the river from west to east, helping the troops surprise Confederate forces at Port Gibson. Steadlily, thoughout the month of May, Union troops worked their way back toward Vicksburg. After a six-week siege, the town finally surrendered on July 4, 1863, ending a decisive chapter in the War Between the States.