mate, in recent ACW mags, there actually have been a few articles & readers' letters discussing the issue of how important guerilla warfare was to both sides during the CW. Some of said articles, in addition to discussing the activities of well-known Confederate guerilla leaders like Champ Ferguson, William Quantrill or Bloody Bill Anderson, also mentioned certain Unionist irregulars who operated in pro-Union portions of certain Southern states such as Appalachian Ala or Tn- regrettably, i don't recall the names of such individuals off the top of my head. Hmmm, maybe if the War Dept had provided more active support to such Unionist guerilla bands- perhaps with the dispatch of more secret agents such as the raiders involved in THE GENERAL train chase ?
Then you also have the OTL issue of pro-Union Cherokee, Chickasaw & Seminole in Indian territory vs the likes of the pro-Rebel Cherokee Mounted Rifles, not to mention the bloody neighbours' war that was Bloody Kansas & Missouri- the pro-Union guerillas/irregulars involved there would be poss candidates for further support of guerilla warfare by Washington too...