Thanks to all for the discussion and the help
Is there any way to lock units in the defensive posture?
I see your point, the answer to my question is no.
I have read all of the wiki, but since I'm new to the game, some of it hasn't sunk into my head and likely won't until I reach some pertinent point in the game where I can put into practice what I've read. Then it makes more sense to me.
Also it helps to read the manual with Adobe Reader and search for the word "offensive"... Substantially identical information is on page 39 of the Rev. 20 manual (specifically AGEod_ACW_ENG_rev20.pdf). A copy of this online manual is located in the .../ACW/Docs/... folder after patching to the latest update version.
I read my manual and refer to it and the wiki often. However, I've not read this rev20 version. Thanks for pointing out that.
In any event, I think that I've figured out what happened. I sent reinforcements from Manassass(CSA controlled) to Harper's Ferry(CSA controlled). But, on analysis of the past turn, I realise that the path that they took ran them through USA controlled Loudon. That must have put them on the offensive and they must not revert back to defensive posture once in friendly territory. They got their heads handed to them on a platter. I went back to that previous turn and re-routed them through friendly territory. Huger and Stonewall just made it in time and Stonewall is now promotable to major general due to his timely arrival and spanking that he administered.