Military service is particularly hard on women.
half of all females who ship out on deployment in the navy do not finish their deployment... for reasons ranging from foolishness, to simply looking for a way out, half end up pregnant and transfered off ship. (ETA- all women in service realize quickly that if the going gets hard, they have an easy out in getting knocked up... its a horrible reason for bringing a baby into this world, but military service puts young and inexperienced women into terrifically distressing situations that lead a lot of them to take this option... )
Just the same... as a father, all you can do is offer the advice of an older, wiser person with her best interests at heart.
Sit her down, and try and get her to explain her reasoning in terms you understand...
Help her by asking her the questions that may not have occurred to one so young.
Such questions as, what if they order you to do something which violates your sense of right and wrong? Does she fundamentally agree with her nation's current military adventures and objectives? Is she prepared for the experience of witnessing friends violently killed or inadvertently killing innocent people?
As she talks it over with you, you will get a sense of whether she has thought this thru or has some unrealistic idea of what she will gain...
If the former, then be proud and let her go, she does not belong to you.
If the latter, then, as her parent, try and explain to her what she is really in for, and offer her alternative means to achieve her goals.