Last updated: 10/24/2010
There is some confusion among reenactors on which tactics manuals were used, and when.
This was the first manual based on Napoleon's new tactics. It was still being used by several units at the beginning of the Civil War, including the Black Hats.
By the 1850's, Napoleon had updated his tactics to make his troops faster and lighter. Hardee wrote a new manual which was approved by a committee that included Gen. Casey. This became the official tactics manual of US regular troops and federalized militia units in 1855.
As far as I can tell, this was simply a redress of Hardee's, removing the author's name because Lt. Col. Hardee defected to the South. (The forward in Hardee's manual was also written by Jefferson Davis.) There are no differences between Hardee's and USIT.
Brig. Gen. Casey wrote a new manual to respond to the fact that Hardee's tactics weren't working. Hardee's manual had been published before the advent of the combat rifle, which utterly changed the tactics on the battlefield. Casey's manual was published and promulgated in 1862.
Hardee also edited his own manual and a new version was published in 1863. Naturally, it was only used by Confederate troops. (If a Confederate reenactor would like to add some more info on this manual, I'd be happy to post it here.)
This is a reprinting of USIT in 1863. I'm not sure why this was done, since Casey's became the official manual for all US regular troops and federalized militia and volunteers, beginning in 1862. As far as I can tell, the 1863 USIT was identical to the 1861 USIT.
After Casey's, the next official tactics manual was published after the war in 1867. This manual contains new features like counting off by fours instead of by twos. This is not a manual for reenactors of the American Civil War.
This page will be updated as I do additional research.