The militia were sent to the vicinity of St. Louis, Missouri, where Brigadier General Nathaniel Lyon dispersed the group using Union Army troops in the Camp Jackson affair on May 10.

Jackson formed a secessionist militia unit known as the Missouri State Guard; he placed Major General[a] Sterling Price in command on May 12.

Brigadier General Ben McCulloch of the Confederate States Army joined Price's militia forces; the combined group defeated Lyon at the Battle of Wilson's Creek in southwestern Missouri on August 10.

In November, while at Neosho, Jackson and the pro-secession legislators voted to secede, and joined the Confederate States of America, functioning as a government-in-exile.

[5] In February 1862, Price abandoned Missouri for Arkansas in the face of Union pressure, joining forces commanded by Major General Earl Van Dorn.

[6] In March, Price officially joined the Confederate States Army, receiving a commission as a major general.

[10] John Newman Edwards, an adjutant serving with Shelby, stated that a stop in the town of Union gleaned many recruits for the unit.

[c] At full strength, the regiment comprised ten companies, all Missouri-raised, designated with the letters A–I and K.[10] In the 1864 United States presidential election, President Lincoln supported continuing the war, while former Union general George B. McClellan promoted ending it.

By the beginning of September 1864, events in the eastern United States, especially the Confederate defeat in the Atlanta campaign, gave Lincoln an edge in the election over McClellan.

[18] On September 27, 1864, Slayback's unit made a minor assault against the defenses of Fort Davidson during the Battle of Pilot Knob; it suffered light casualties.

[27] Needing supplies, Price ordered two side raids, one of which targeted the town of Glasgow; the other was a thrust by Thompson towards Sedalia.

[32] Slayback's battalion quickly found an alternate ford, and crossed the river, attacking Colonel Charles R. Jennison's brigade in the flank.

[35][36] After being pressed by a Union attack, Shelby then ordered Thompson to charge, and the cavalrymen, including Slayback's battalion, were soon engaged in a melee.

[38] After a Confederate disaster at the Battle of Mine Creek on October 25, Shelby's division, including Slayback's battalion, were recalled from detached duty to serve as a rear guard for Price.

Slayback's unit was initially posted on the Confederate left, with the intention of threatening the Union flank, but Price ordered the rear guard to fall back to behind the line of the Little Osage River.

Shelby's division fought by forming a series of weak lines, each briefly holding up the Union pursuit, and then falling back some distance in turn; these tactics gave Price some space to continue his retreat.

The rear contained disorganized elements of the divisions of Marmaduke and Major General James F. Fagan and the brigade of Sidney D. Jackman, as well as Collins' Battery.

After 15 minutes of fighting, the Confederate line, including Slayback's unit, withdrew in a fashion Shelby described as "melting away".

Despite repulsing the Iowans, the Confederate assailants came under Union artillery fire and broke off the attack, bringing the Battle of Marmiton River to an end.