Nathan Bedford Forrest was carefully studied by the German High Command and especially influenced Rommel. The US army doctrine of combined arms highly mobile offensive strategies have their origins in Forrest’s tactics and campaigns. It is unfortunate and somewhat blasphemous that ISIS is benefitting from his legacy. His greatest strategic failure however is that by late 1864, he was unable to deploy a significant force out of northern Alabama to disrupt Sherman’s rear and supply lines.
Please consider that in 1864 when a large well-equipped force drove south from Memphis, Forrest lured them further and further south with a small portion of his command. He then raided the Federal Headquarters in Memphis itself:
http://en.wikipedia.org/wiki/Second_Battle_of_Memphis
Been introducing my nine-year-old to Forrest over the past month - It's been fun to re-visit the most interesting character that war produced.
I would say the onus for the strategic failure in 1864 lies with John Bell Hood. Forrest was not the theatre commander, Hood was. Forrest knew he was subordinate to an idiot, a man way over his head.