The reason to do this is because smaller units were sometimes detached from larger units for special operations. A company may be detached from a regiment, a regiment from a brigade and so on.
If you don’t follow the smallest possible unit, you may spend your time researching a battle, only to find later that your ancestor’s company or regiment was elsewhere at the time.
This happened with William B. Poore’s 16th Mississippi Infantry Regiment in October 1863.
On October 9, at about the time Francis M. Poore and John F. Poore had settled down for the siege of Chattanooga, General Robert E. Lee put their younger brother William on the march. Lee sent William and the rest of Lieutenant General A. P. Hill’s Corps and that of Lieutenant General Richard S. Ewell out to strike Union Major General George G. Mead.
On the march toward Mead, while the rest of Brigadier General Carnot Posey’s Brigade went with Anderson’s Division toward Greenwich, Va., William and his 16th Mississippi comrades stayed at Buckland to support rebel cavalry and to keep the Yankee horse soldiers from attacking the main Confederate column.
In Buckland, William and his fellow Southerners fought Union horse soldiers from behind fences and houses. The 16th Mississippi eventually drove the federal cavalrymen across Broad Run and forced them to retreat.
The 16th Mississippi now headed to Bristoe Station to rejoin the brigade. As they neared Bristoe, the sounds of heavy musketry and artillery fire drifted their way. A major battle was already under way. But by the time William and his comrades arrived on the scene, it was dark and the battle was over. Hill had led William’s comrades in the other regiments of the brigade into a disaster.
The rebel casualties included Posey. An artillery shell had hit his thigh and he later died from the wound.
Because the 16th Mississippi and had been detached from the rest the brigade, William’s life may have been spared. Such are the twists of fates of war and can make for stories as interesting as a battle account.