Should i remain loyal?

I am presented with a dilemma. I am part of a lucrative military free trade agreement. My ally and i enjoy large supply trade and are making excellent progress in securing the last enemy controlled area, but as the divisions are moving i am thinking of attacking him, a victory against my unsuspecting ally would give me strategic dominance in the lake network in which we are operating. The tactical situation is: He has a bigger airforce but not by much. My land army vastly outnumbers his, as does my navy which has two aircraft carriers loaded with f14s and a large escort fleet to shell his base. A surprise attack after a large build up in his base would almost certainly yield my desired response although i feel somewhat cowardly attacking him. What do you recommend?