At the Battle of Grand Gulf on April 29, the unit helped repulse a Union Navy attack against the Confederate defensive works.

After elements of the Union Army of the Tennessee landed below Grand Gulf, the regiment fought in a delaying action at the Battle of Port Gibson on May 1.

After the Confederate defeat at Port Gibson, the 3rd Missouri Infantry Regiment was engaged at the Battle of Champion Hill, another unsuccessful attempt to stop Major General Ulysses S. Grant's advance against Vicksburg, on May 16.

The regiment was routed the next day at the Battle of Big Black River Bridge while serving as part of the rear guard.

Brigadier General Nathaniel Lyon of the Union Army drove the secessionists into the southwestern portion of the state,[1] but was defeated and killed at the Battle of Wilson's Creek in August.

[a][6] On February 12, 1862, the regiment left Springfield for Cove Creek, Arkansas, as part of a Confederate withdrawal caused by Union pressure.

On March 3, the regiment left Cove Creek as part of a Confederate advance against a Union force that was in the Pea Ridge, Arkansas, area.

[7] On the morning of March 7, Price's division, which contained Little's brigade, encountered Union troops near a position known as Cross Timber Hollow.

At one point the fighting, the 3rd Missouri Infantry, led from the front by Rives, initially drove the enemy back before becoming disorganized and halting.

[9] On March 8, a Union cannonade forced the 3rd Missouri Infantry back from an exposed position in an open field to a more protected one in some woods.

In late May, the Siege of Corinth ended when the Confederates evacuated the town, and the 3rd Missouri Infantry spent the summer stationed at various points in northern Mississippi.

[17] At the Second Battle of Corinth on October 3 and 4, the 3rd Missouri Infantry was part of Colonel Elijah Gates' brigade of Brigadier General Louis Hébert's division.

[19] However, even with Gates' brigade in good shape, Price determined not to attempt a large-scale assault against the interior Union lines on the 3rd.

On April 5, the 3rd Missouri Infantry, as part of a larger force, was moved across the Mississippi River into Louisiana,[27] where they encamped near Bayou Vidal in Tensas Parish.

[27] At the Battle of Grand Gulf on April 29, the 3rd Missouri Infantry defended a line of rifle pits that extended between the two main Confederate fortifications.

Brigadier General John S. Bowen, the Confederate commander at Grand Gulf, responded to the Union landing by sending a force to Port Gibson, Mississippi to intercept Grant.

[37] A canebrake provided cover for the Confederate attack,[38] which fell upon a Union line composed of the brigades of Colonel James R. Slack and Brigadier General George F. McGinnis and five artillery batteries.

While making the necessary movements, some of Pemberton's force encountered elements of Grant's army on May 16, starting the Battle of Champion Hill.

[44] Later in the battle, the Confederate left was severely threatened by Union assaults, and the First Missouri Brigade was sent to shore up the failing line.

[46][48] The entire brigade then charged the main Union position, capturing a crossroads and Champion Hill, two key battlefield locations.

However, a Union charge broke the Confederate line and routed the defenders in the Battle of Big Black River Bridge.

This forward position was determined to be dangerous due to a large ravine between it and the main Confederate line, so it was abandoned on the 19th.

[54] On May 22, the men of the regiment manned the Confederate line at a point known as the Stockade Redan, where they, as well as other elements of the First Missouri Brigade, fought off Union attacks against the position.

In February 1865, the regiment was transferred to Mobile, Alabama, adding to the city's defenses, where it surrendered on April 9 at the Battle of Fort Blakeley.