He attained the rank of brevet Brigadier General, United States Volunteers before leaving military service.

[2] Dressed in his Mexican–American War uniform, David Moore rode into Alexandria, Missouri, on June 24, 1861, to take the oath of loyalty to the Union.

Given the rank of captain, he had handbills printed the same day inviting "all who are willing to fight for their homes, their county, and the flag of our glorious Union" to enlist "bringing their arms and ammunition.

[2] After the swearing in and organization, Moore and about five hundred men moved from Kahoka to the strategically important river port of Athens, Missouri.

Also in Clark county and not far from the confluence of the Des Moines and Mississippi Rivers, Moore established a training camp there as well as used it as a base of operations against area pro-Confederate bushwhackers and Missouri State Guard (MSG) under Colonel Martin E.

[3] Athens also had the advantage of being across-river from a Union army supply depot at Croton, Iowa and its railroad access.

[3] On July 21, 1861, Moore's troops, with assistance from Illinois and Iowa units, attacked the village of Etna in next-door Scotland County, Missouri, and drove off elements of MSG cavalry then retreated back to fortified Athens.

[2] Moore and his regiment wrapped up their busy 1862 by participating in the first phase of General Grant's Vicksburg Campaign in December.

The year 1864 would see the men of the 21st once again assigned to hard fighting as they served in the Third Division of the Union XVI Corps participating in the Meridian Expedition, Red River Campaign and, especially, the Battle of Tupelo, where Moore and his men were noted for their fierce stand against the dismounted cavalry of Nathan Bedford Forrest.

His first wife, Pennsylvania native Diademia (Schnabel) Moore, died in 1865 after bearing him five sons and a daughter: William W., Eugene, John C., Charles A, Thomas, and Frankie.

David Moore's second wife, the widow May (Mattingly) Carnegy of Union County, Kentucky, bore him three daughters—Katie (deceased in childhood), Katie D., and Nellie—and also brought two step-sons and four step-daughters to the marriage.