This is going to sound harsh, but believe me, I'm not judging...just trying to help.
The grown ups in this scenario need to see things from this girl's point of view. Dad and Mom split up, Dad leaves and starts a new family with someone else, then Dad goes off to Iraq and may never come back (at least that's true from her perspective, even if he wasn't in real danger, which is never the case in a war). Wow, let's just put a stake in her heart!
Is it any wonder this girl is looking for some adult male attention and affection? Who wouldn't be?
I think your husband and his ex need to put aside their differences, no matter how big, and concentrate on this girl. It should start with a great big "we're sorry". We're sorry we trashed your life. We're sorry we all acted selfishly, getting what we wanted and ignoring what was best for you. We're sorry we put ourselves first, when you should have been put first. We're sorry we expected what makes us happy to make you happy. We screwed up. We made a mess of your life and you have every right to be angry and to hate what we've done to you.
Your husband should be devoting tons of time to this kid, even if it takes time away from the others. She didn't ask for that family, but she's expected to be just thrilled about it nonetheless. Is she expected to look after the new kids, like she's a built in babysitter? Imagine if you were 15 and you were being told that you had this new responsibility that you didn't want, didn't ask for and didn't have any part in creating. Wouldn't you be angry and resentful? Dad's new family is now your responsibility while Dad and his new wife go out and have fun. I think I'd be a little peeved too.
I think you should sit down with her and apologize too. Tell her that you too acted selfishly, getting what you wanted at her expense (I know it's hard to see it this way, but it's true--this kid lost her family and you gained yours...and it was at her expense.) She was Daddy's only girl, and now she has to share him. That hurts. She's looking for a way to fill that void.
Tell her that you can't and won't be a parent to her. She has two parents and doesn't want or need a third. Tell her that you can be a confidante and a friend and that you hope that the two of you can build something new and good out of this mess. Earn her trust keeping her confidences--don't share them with your husband. Let her know that you want to trust her and then give her chances to earn your trust.
Make and allow time for your husband to be with her alone. She needs her father's attention solely focused on her. It's going to mean less time with you and your children, but he owes this kid a huge debt. Let him pay it.
Lastly, don't blame her for being normal. Everything she's doing is absolutely normal behavior for a kid who's had her world turned upside down. This kid's heart was broken! Dad's off with his new family, or worse, in Iraq. Mom's probably either trashing Dad all the time or off socializing or both. She's alone and hurting and scared and angry and she has every right to be all of those things. She doesn't need to be fixed. She needs to have the attention she has every right to and isn't getting.
I know that this sounds brutal, but it's meant to be helpful. You seem to be genuinely concerned about her and that's a big plus, but you also have to look inward and accept responsibility for your part in the mess and also see things from her point of view. She deserved better than she got.