Fighting wars is defense, but so are preemtive strikes, etc. And one might say that the best defense is a good offense, so I'd still say the main
purpose is defense. I'm not saying other things can't be included too, just that this is the main one in today's age. In America, the military falls under The Department of Defense
. There's a reason for that.
Archer said that they were going to need all of the muscle they could get when the Admiral wanted to send troops along. He didn't feel that way at the start because he didn't imagine Earth would be preemptively attacked with 7 million dead from the Xindi threat in the Expanse. After the attack on Earth, it was time to get into the fight, and it was a war. So, Archer's position changed from more the benign explorer, to leading a defensive assault.
I don't know if Malcolm's "fuzzy writing" is by accident. My guess is it's not. I think that he probably didn't want to serve in the traditional military, but because the tradition was in his blood, he perhaps wanted to explore a new frontier--space. To really do that and provide some kind of military style help, he needed to join Starfleet. I think that for Malcolm, it's a good mix.
It may have been the perfect time to go over things things in detail on camera, but that's not what the focus of the series seems to be. I'll grant you that this could be a part of the problem direction-wise with the series, but I can't have a full opinion on that until I've seen the whole thing. So, further discussion on this with me will have to be a topic for another day.