I'd be absolutely SICK with worry, but fact is that at 18 there's not doodly you can do about it.
Any more than you can stop her from marrying a jerk...
Or deciding to move to Alaska...
Becoming a vegan...
Joining the Peace Corps...
Becoming a missionary...
I've heard some horror stories about kids that signed on and had no idea what they were in for. Kids that are now grown and scarred for life. Kids that still haven't been released after they've served the 3 or whatever years they agreed to. Kids that can't sleep at night for the nightmares. Kids afraid to marry for fear they'll hurt their spouse/kids. There's a LOT that can go wrong, so I understand completely your fears, but those kids' parents couldn't do squat to stop things, and neither can you... unless of course you're a closet billionaire or are part of a huge political family... then I suppose you could buy a desk post for your child... but for the rest of us we just have to hope for the best and be there to support her should any of this turn out not to be all roses.
In your shoes I would find as many FACTS as I could... and I wouldn't limit it to the current situation, but would show what the Navy/Army/Air Force have done in the past. I'd show her exactly what kind of horrors she can expect... exactly what she may be ordered to do. I know that I do not have the stomach for killing, that's just a fact. In the heat of the moment, if I was threatened, or my child, then maybe I could kill to protect myself (thankfully never had to try) but to be ordered to kill... perfect strangers... and any children nearby are just collateral damage... there's just no way I could do it. Thus there is no way that I would EVER sign on to any military group. Unfortunately many kids don't realize that that is a real possibility... maybe they think that's a very rare thing... or that their unit wouldn't be in that... or maybe if they're on a boat it distances them from the village they're bombing since they won't see the bodies. I really don't know but there are SO many cases of kids that got a rude awakening when faced with the facts. And then they can't leave, they are forced to continue doing these things that make them ill. It takes a part of your soul that you can never get back. Even if you aren't the one pulling the trigger, knowing that you loaded the gun has broken many a soldier. In wars throughout history the highest casualty numbers are often the innocent bystanders... the soldiers are trained to get out of the way and are only hurt when they make an error... it's easy enough to find those estimates... I say estimates because they rarely bother keeping an accurate count since those children are just collateral damage and not worth a second glance. I would have no choice but to show my child these FACTS so that at least I know they'll be making a truly informed decision... not based on one sided rose tinted information.
Sorry for the ramble there, but war is something that has always hurt to even think of. The men and women who're sent in, they are the ones who end up scarred, not the politicians sitting on the hill eating a thousand dollar lunch paid for by our taxes... I think if those politicians were to actually SEE what their half researched decisions actually caused then maybe they'd be a little more supportive of our troops and wouldn't send them into some of the places that have turned out to be disasters.
If this is off topic feel free to delete the whole mess, I don't mind at all, but I think that the recruiters should be a LOT more honest than they are and the only way that will ever happen is by spreading the FACTS.