Probably true. The Boer War I think is a good insight into the tactics The Union might have had to use had The South not surrendered and instead adopted a guerrilla warfare approach.

Britain defeated the Boers in conventional warfare in the early 20th century, however the Boers then adopted commando raiding tactics which were highly effective.

Eventually Britain brought in a scorched earth policy and concentration camps to isolate the insurgents and starve them out.