Well, there are a couple ways we can look at that.
One, defense can easily be extended to allies, or allies' allies, which would involve an offense of our own. In most cases I'm aware of, this is considered legitimately defensive if done via a treaty. The question of defending an ally who started a war would be another one, though, which I'm certain would need to be decided on a case by case basis, considering the strength of their CB and any moral factors.
OOC: Two, from a game-play perspective, we still want to win, right? Offensive wars are a part of the game, and they make the game fun. Look at it from the standpoint of a chess game. It wouldn't be very fun if one player never moved his pieces and never tried to take your pieces. It's in everyone's best interest, including our potential enemies, that we play like any other alliance as far as war goes, because war is what makes the world go round. /OOC
But no, our organization has never actually started an aggressive war in our eight and a half year history. (Have we fought in them? Yeah.) Not saying we won't on this planet, because its not the same situation, but it's a track record, whether one finds it impressive, boring, or any other combination of emotions.
So, all that to say, you may be disappointed if you're looking forward to us only declaring strictly defensive wars.
It's far more likely that we'll only declare Strictly "defensive" wars.