Yeah, totally agreeing with Medaforcer here. A persons love can be expressed in many ways, and this is one of them. It's not an act of selfishness if you don't want your partner to join the army, just an act of concern.
It's difficult to offer advice here, because it's a serious issue and we here on the forums don't really know the full story behind what's going on. If you are concerned for her safety and have properly considered the entire situation (as in, read up on how well people are treated in the army, the chances of her going off to fight in a war, etc), then by all means try to convince her to stay. However, there is a boundary here- if she is absolutely adamant about this and won't change her mind no matter what you say, there's little use in protesting, and you'll just have to suck it up, support her, and be darn proud your girlfriend is brave enough to do this.