First of all, trying to raise the age to fight in the military is idiotic. If one can vote, they can support what they vote for. The age to enlist is just fine like it is. By the way, you can't enlist when you're 17 unless you have permission from parents, so that argument is moot anyway.
I can see the logic in arguing that if people are old enough to be in the military, they're old enough to drink. However, you've also got to consider that serving in the military reflects a certain amount of honor and responsibility, a responsibility and maturity that quite frankly usually aren't present in the typical 18-year-old. So I can understand being legally able to drink if you ARE in the military, but there are even more arguments to be made against the legal drinking age bring brought down to 18 across the board.