I don’t advance him as “most underrated of all time”, but I think two “most underrated of the Civil War” would go to:
Union:William S. Rosecrans. He picked up command of the Army of the Cumberland after Buell had loused things up; Rosecrans went on to win a brilliant victory against Bragg at Murfreesboro (in theory, he lost the battle, but Rosecrans refused to retreat, so Bragg assumed that he had lost the battle and made a hasty retreat) and chased the Rebs valiantly and quickly.
Unfortunately, at Chickamauga, Rosecrans made the mistake of trusting the judgement of Ambrose Burnside and his own fears, and so beat a hasty retreat in a battle he was actually winning. Rosecrans was removed shortly thereafter, and Sherman given command. And Rosecrans faded from public memory.
Confederacy: Joseph E. Johnston. Won the First Battle of Bull Run (although Beauregard got the credit as the ranking officer, it was Johnston’s leadership on the field- and his brilliant manuevering in getting his troops to Bull Run while leaving a Union army twice his size off in Harper’s Ferry waiting for his attack) and was given command of the new Army of Northern Virginia. He subsequently lost this command by having the dual misfortunes of being wounded at the battle of Seven Pines and being replaced by somebody by the name of Robert E. Lee.
Johnston was eventually given command of the Western Theatre, but not until Pemberton had been blocked up in Vicksburg and it was too late to make an attempt to save him. Johnston eventually took direct command of the Army of Tennessee, and fought a brilliant campaign of delay against Sherman. Johnston realized his army was too weak to defeat Sherman in battle, and so Johnston waged a campaign of manuever and retreat which bogged Sherman down tremendously. Unfortunately, President Davis decided that what the South really needed was a decisive victory, and so replaced Johnston with John Bell Hood, saying, “This man will fight.” And Hood lived up to that by leading his troops into a direct attack upon Sherman- an attack they lost heavily and took massive casualties in, basically breaking the Army of Tennessee as a fighting force and allowing Sherman to walk into Atlanta (which, quite possibly was the event which assured Lincoln’s re-election).
Eventually Johnston was given command of a new army in an attempt to delay Sherman’s march through the Carolinas; Johnston again tried to use delay and retreat tactics, but Sherman outnumbered him so much that it was pointless, and Johnston surrended around the time that Lee did.
Joe Johnston is mostly forgotten when talking about great Confederate generals; but he was the one who came closest to actually winning the war for the South.
JMCJ
“Y’know, I would invite y’all to go feltch a dead goat, but that would be abuse of a perfectly good dead goat and an insult to all those who engage in that practice for fun.” -weirddave, set to maximum flame