You joined the Army without telling your girlfriend. What now?

“I signed up to join the Army because I want to give back and much of my family has served. I’ve been with my girlfriend for three years and when I told her that I signed up, she became really mad and she told me that she doesn’t want me doing it.
She’s concerned about my safety and being gone for a long time.
I probably should have talked to her about it first. What can I say to make her feel comfortable with it?”