The Union assault on the Confederate stronghold at Vicksburg began May 1, 1863, with the Battle of Thompson's Hill. A number of other battles throughout May pushed the Rebels back into Vicksburg on May 19. Gen. Ulysses S. Grant made two assaults on the Rebel's defenses, the first on May 19 and the second, more organized, on May 22. Failing to take the city by brute force, Grant settled down to a conventional siege, the city now being cutoff from all sides with no opportunity for fresh troops, food or other supplies.

The 16th Ohio played a major part in the two assaults on Vicksburg and then on the siege line until June 8, when it, along with the Ninth Division, was ordered to move about 10 miles east, back to the area of Big Black River Bridge, where it would guard the eastern approaches to Vicksburg from Confederate General Joe Johnston.  Johnston was in the area and looking for a way to break the siege and relieve Vicksburg. Eventually, nearly half the Union forces at Vicksburg would be stationed to the east, guarding against what seemed to be a highly likely attack by Johnston.