We struggled endlessly with this question during the Vietnam era and the draft. Each person has to find his or her own answer, using his or her own moral compass. No one else has a "right" answer that is more powerful or compelling than your own, and you have to live with yourself. At least we aren't having a draft these days -- you've got more breathing room than young men had back then (women were not drafted, though they could enlist).
To balance the implications of joining the military versus your own moral code, it might be helpful to verbalize your personal moral code as a beginning step. Then start asking some questions in light of the standard you have stated. You can always backtrack:
1) Do you question the mission of the Air National Guard overall, or even just during the period of time you would be a member? Consider how they define their mission and the exact activities that you might be required to perform.
2) Is there another form of national service you could perform that you would find less questionable, if your doubts are related to militarism in general? (this could be official service or unofficial)
3) Would you find some activities of the ANG acceptable to you and others not? Is there a way to sign up for something you'd accept? Would you be OK with supporting other people doing things you find objectionable, while not doing them yourself?
4) If you are considering signing up for personal financial reasons, what other ways can you find to meet those?
5) If you are considering signing up for career reasons (say, to learn to be a pilot) but have doubts about militarism, what other ways can you find to meet your career goals?
6) Do you feel your own moral code is more important to you, equally important, or less important to you than the principles under which this country maintains a military force?
7) Do you see your personal moral code as something that might change in the course of your life, or as something immutable?
8) Would you struggle every day with the social aspect of the military (being around people you strongly disagree with, taking disagreeable orders, dealing with a great deal of structure and bureaucracy, etc), could you put your personal feelings aside during your period of service, or would you actually find those aspects a welcome challenge?
Add any other questions that address your situation.