And yet...
Sherman versus Johnston is a complicated question and both commanders deserve praise and criticism for varying aspects of the campaign. Against Hood, though, Sherman performed poorly. In the last phase of the Atlanta Campaign, the Confederates lost much more than the Union won, because Hood was a walking disaster as an army commander and did pretty much everything wrong. Had Sherman been a more competent commander, Atlanta would not only have fallen but the Army of Tennessee would have been destroyed.
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Sheridan and Sherman were both overrated for reasons I have already described. Neither Canby or Ord had seen all that much combat; Canby had in New Mexico and Ord had at the Battle of Hatchie's Bridge, but both had otherwise been mostly on the sidelines. There was not really anything to suggest that they had "potentially great army commander" stamped on them. I'm rather bemused that you would include them on this list.
When the 1864 campaign opened, critical commands were given to Benjamin Butler, Nathaniel Banks, Ambrose Burnside, Franz Sigel, and David Hunter. Why did these recognized incompetents receive such commands if the Union army command was by then so outstanding, as you allege?
And Jubal Early, Richard Taylor, and John C. Breckinridge, all of whom really came into their own in 1864 and demonstrated high effectiveness in independent commands. Hardee demonstrated an untapped potential for independent command around Savannah in late 1864. John B. Gordon was just then coming into his own as well. You're oft-repeated contention that Lee was the only solid independent Confederate commander is simply wrong.
And yet, Sherman beat Johnston and Hood (the best the rebels had, apparently, for the Georgia front, at least in the eyes of Jeff Davis) from one end of the state to the other, captured and destroyed Atlanta, and cast off for Savannah, and the rebs' best response was to send Hood into Tennessee...
So yeah, Sherman was a horrible army commander.
Canby and Ord both had something damn few rebel generals managed to demonstrate: the confidence of their superiors, and the ability to manuever and sustain a field army in an offensive war of movement and in the enemy's country in 1864-65. In comparison, Early's efforts were a) a raid, or b) a failed defensive campaign.
The 1864 campaign opened before the election; political generals were still important. Compared with Polk and some of their rebel equivalents, they certainly don't look terrible; hell, Banks' beat Jackson, of all people, at Kernstown in 1862 pretty handily.
- Early led a raid that failed and fought a defensive campaign that failed in 1864-65;
- Taylor managed to beat Banks on the Red River;
- Breckinridge fought a couple of minor defensive actions in East Tennessee/West Virginia in 1864 and then was named secretary of war; his most significant contribution to the war was getting Northrop removed;
Throw Hardee (who had been relieved, essentially, along with JE Johnston, of course) and Gibbon into the mix; it still comes down to - maybe - three army commanders that met Davis' standards in 1864-65: Lee, Hood, and Kirby-Smith, and we all know how things worked out with them...
The US had - at least - three times as many able general officers capable of commanding independently at the (roughly) army level.
Put it this way, if you wish: the rebellion began the big war in 1861-62 with - maybe - four army-level commanders: Lee, JE Johnston, AS Johnston, and PGT Beauregard. When it ended, four years later, the only two forces that mattered in terms of rebel armies east of the Mississippi were commanded by Lee and JE Johnston.
In the same period in 1861-62, the US army-level commanders included McDowell, McClellan, Pope, Buell, Rosecrans, Halleck, and Grant. Four years later, the only one of them in active combat command was Grant. In 1861-62, other than Sherman's short assignment to relieve Anderson in Kentucky, all of the other US army group/independent army-level field commanders - Sherman, Meade, Thomas, Sheridan, Ord, and Canby - were men who had been brigade or, at most, division commanders in 1861-62.
Which speaks to the richness of the US' bench, and acknowledgement of the very real military advantages the demographic differential (4 to 1, after all) gave the US war effort.
Best,