Upvote:4
The distinction appears to have been a ruse by Rosenkranz to entice the selection of good men, instead of rogues, to fill the initial assessment from the regiments. In his History of the Pioneer Brigade Geoffrey L. Blankenmeyer notes:
General Rosecrans' original order of formation inferred that the men selected for pioneer duty would stay with their mother regiments during a march and that he would detach them when they were required for pioneer duty. At least that is what the General originally communicated to the regimental Colonels. However, this expressed intent may have been a ruse "intended to ensure that the regiments detailed their best men and not their dregs" for this detail. The Colonels did indeed detail their most talented men to the Pioneers. However, it was discovered soon thereafter that the original core of Pioneers would become a separate unit permanently detached from their former comrades. The result was the development of a lasting bitterness among the regimental brass for Rosecrans' brain-child. The bitterness within the brass began as soon as this news was announced and lasted for the duration of the unit's existence.
Within two months of the Pioneer Brigade's initial creation it had been amassed and undergone 3-4 weeks of specialized training; and then returned to the Army of the Cumberland in time for the Battle of Stones River. There, as a reserve brigade, it stemmed the initial Rebel onslaught, buying time for Rosenkranz to rally his force and recover a victory.
This is supported by a quote in that history from Philip L. Shiman1 supporting this speculation.
Every regiment had its share of shirkers as well as active, intelligent soldiers. Naturally, (regimental) colonels looked for every opportunity to get rid of the no-good soldiers. Had they known that the men assigned as pioneers would be put permanently in a pioneer brigade, they would have chosen their worst soldiers. Realizing this, Rosecrans told the colonels that the pioneers would stay close to the regiments and provide pioneer support to them directly. The colonels therefore selected their best, most talented men, in the expectation that they could use their pioneers as they saw fit. Therefore, (history) should consider it quite a compliment. that (an individual) should have been selected for the Pioneer Brigade. It meant his colonel thought highly of him.
More on the History, organization, and Battles of the Army of the Cumberland available here in Vol. I and Vol. II of Major General George H. Thomas' History of the Army of the Cumberland (1875).
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