A civil war army on the move uncoils and stretches out like a slinky. The rougher the road, tighter the terrain and longer the march, the more attenuated the force becomes — “When these marches commenced, the men would be in regular military order, four abreast; but the first half-mile usually broke up all regularity.” 1 Infantry units would typically march in rows of four with space left between regiments and on a steady march might cover 2.5 miles per hour. Thus it can be imagined that even without artillery and wagon trains a division of 4,000 infantry on the move would stretch out over more than a mile and maybe as much as two miles of road. As a result the time at which the front and the rear of a column reached the same destination could be separated by an hour or more. On narrow country roads with uncertain conditions and unforeseen incidences, delays occur and separations appear between units, reducing speed and increasing dispersal. In short, movement created disorder that was only dispelled by stopping. These factors should be kept in mind when evaluating a civil war army on the move.
Consider General McPherson’s march through Snake Creek Gap to Resaca, Georgia in early May 1864. It is sometimes claimed that McPherson had a 4-to-1 or 5-to-1 advantage over the Confederates in Resaca. This is a superficial view that counts his entire command as if it was all concentrated in front of Resaca.
McPherson led a portion of the Army of the Tennessee that consisted of five infantry divisions – 2 from the 16th Corps under Dodge and 3 from the 15th Corps under Logan. At dawn on May 9th, the lead unit of Dodge’s corps emerged from the eastern mouth of Snake Creek gap. The rest of the army was stretched back through the gap, campsites spaced out along miles of road. As the morning progressed Dodge’s two divisions would have been stretched out with the lead unit half way to Resaca before the last unit cleared the end of the gap. From the eastern end of the gap to Resaca was about 8 miles but the terrain was rough and confederate cavalry harassed the movement, slowing it down. By the afternoon Dodge had his command gathered in front of Resaca but now Logan’s corps was stretched out along the road with the tail still in the gap. As the afternoon slipped away, the front of Logan’s first division was just starting to arrive. It would take time to bring it together and hours before the other divisions could get there, even if they were called forward since someone needed to guard the wagon train, the gap and the maintain the connection with the rest of the US forces. Thus while on paper McPherson might have had five divisions under his command, at the critical point he had at best just three with which to act. His decision should be considered in that light.
One additional observation: part of the point of the above is that the route to Resaca through Snake Creek Gap was capacity constrained. After he occupied it, McPherson allocated men to improving the roadway to increase its capacity. But prior to this it was a narrow, limited roadway that could only accommodate a certain quantity of marching men per hour. As such, beyond a certain point increasing the size of the force sent would not have increased the throughput. So if the larger Army of the Cumberland had been sent instead, the size of the force that would have reached Resaca the afternoon of the first day would have been roughly the same. The main difference would have been the size of the traffic jam in Snake Creek Gap.