Actually, it's more #3.
Troops stand down when they have a period of relaxation after an alert or having been on patrol. In this case, the Iraqi unit where pulled out of active duty - ordered to stand-down - pending an investigation into the shooting. They need to find out how friendly fire caused this death -how did this Iraqi solider mistake the US soldier for the enemy? - and until they do, if these Iraqi troops are still on active duty, what's to stop the same thing happening again?
Your thorough explanation substantiates the indispensability of my inquire. I stand on my ground that on this place we might use each of them (Nr.1, Nr.2 or Nr.3). Their meanings are closely and interwoven connected each other.
Thank you for your attention and readiness to help others.
Thank you for your pressing particularization. I make no objection to your statement. But we have to try to see through the words and phrase.
There are more lines of the mentioned above article.
"The investigation by American and Iraqi authorities may renew longstanding question about the loyalties of Iraqi forces, who are supported to assume control as American troops withdrew."
"withdraw" = not "go off duty"
In my opinion, one thing is 1. to leave a witness stand (to leave for short time, as long as continues a investigation); quite near to that is 2. withdraw-to take back or away from the duty transitory as above at 1. and there is quite different (your variant) 3. go off duty (demobilization) (withdrawal of a military presence).
In the case of your variant (Nr.3), the American troops have come to stay
in Iraq till to the Second Coming.
In this context, the unit concerned does not "withdraw" but is/has been actively ordered out of service. It is particular to the situation and nature of the event.
I don't understand your point about the Forces and their term of service. It is irrelevant.
The report is quite clear that investigations are taking place and until the military have finished these and concluded what in fact happened, the unit concerned has been ordered [taken] out of active service.
The point about the forces, which you called irrelevant, actually is th point appointed from the article's author, who is an American. A few words for your information,: the trained Iraqi forces have to change the American troops as they would be withdrawn later from Iraqi. If there is a never-ending stream of alternating demobilizations (go of duty) of different units of Iraqi forces, which are supported to assume control over Iraqi, then who will change the American troops if all disloyal units of Iraqi forces were demobilized.
Please, excuse my frankness. I see, you manage English perfectly, but you are remote from military affairs and politic. Let things take their course. Our immediate goal is steadily adoption of English language.
Thank you for your tenaciously patience and marked attention towards my humble person.
Against my will I agree on all important points referring the matter of interest to me, especially the meaning of the idiom "stand down." By your permission I would attract your attention last towards the meaning of "stand down" (n), namely "a relaxation from a state of readiness or alert." I was mislead from this meaning. That one was grounds for my long-winded inquiries.