On May 17th, Cockrell's brigade was part of a force defending the crossing of the Big Black River. It was on the Confederate right, between a body of water named Gin Lake and Brigadier General John C. Vaughn's brigade. The 5th Missouri Infantry was positioned to the right of the 4th Mississippi Infantry Regiment. While the 5th Missouri Infantry repulsed a Union probing attack, a Union charge quickly routed Vaughn's brigade, exposing the flanks of the rest of the Confederate line. The Missourians were unaware that Vaughn's line had broken, and were surprised to receive orders to retreat. After the 4th Mississippi Infantry routed, the 5th Missouri Infantry was in a very exposed position and also routed, with the men racing across the river crossing. After the defeat at Big Black River Bridge, the 5th Missouri Infantry entered the defenses of Vicksburg, Mississippi, where it was part of the Confederate reserve. On May 19th, Grant's Union army made a major attack against the Vicksburg defenses. The 5th Missouri Infantry and the 1st and 4th Missouri Infantry (Consolidated) were sent to support a point known as the Stockade Redan, which was defended by the 36th Mississippi Infantry Regiment. The two Missouri regiments drove back the attacking Union soldiers after an encounter Bevier described as a "sharp conflict" and "dreadful slaughter". After the Union attacks were repulsed, most of Cockrell's brigade returned to reserve duty, although the 5th Missouri Infantry and the 1st and 4th Missouri Infantry (Consolidated) remained on the front line.