In a previous post I wrote about the impact of movement on the organization and strength of a force and used the advance of the Army of the Tennessee through Snake Creek Gap and to Resaca Georgia on May 9, 1864 as an example. There is another aspect of that movement I wish to explore. I have seen writers blithely stated that McPherson moved with 24,000 or even 25,000. The truth is that the size of the force was smaller that day.
In an earlier post I wrote about how incremental detachments reduced the available force at Cedar Mountain well below the strength report at the beginning of the month. A similar phenomenon was at play with the Army of the Tennessee on May 9th 1864. Initially McPherson had almost 30,000 men based in northern Alabama. Specifically, Logan’s corps occupied the Memphis and Charleston railroad from Huntsville to Stevenson while Dodge’s command held the Nashville and Decatur railroad from Decatur to Pulaski, Tennessee.  Sherman’s original plan had McPherson advancing directly from this area toward Rome, Georgia.  But at the end of April he revised the plan and directed that McPherson shift his troops to Chattanooga so that they could advance in conjunction with the other armies moving into northern Georgia.  Repositioning these forces could uncover the critical rail lines, so detachments were assigned to remain in northern Alabama and the size of the initial force that McPherson would bring was reduced to 20,000 split evenly between Dodge and Logan. 
Other temporary detachments also occurred. For example, the 7th Illinois, patrolling the Tennessee river near Huntsville, was not relieved right away and had to catch up later.  Two regiments holding a pontoon bridge over the Tennessee river were held back until the bridge was taken up.  Two sections of the 1st Iowa battery were delayed by lack of horses. The 37th Ohio was delayed by a rail accident.  These units would rejoin the army but not until after the initial attempt on Resaca.
Finally, on May 9th, as the army moved through Snake Creek Gap, the wagons and other baggage were left on the west of the ridge with two brigades and a battery to protect them.  As noted previously, about of third of the moving column never reached Resaca as it was stretched out along the road from the gap. So rather than the alleged 24,000 men, McPherson had perhaps half that on hand with which to confront the Confederates at Resaca the afternoon of May 9th, 1864.