I already mentioned this on LINE today, as long as you was there when War search began and match has been made, you can leave your alliance go over to your friend's alliance, get some Space Age troops, then go back to your alliance and attack in War.
I and the Loonies Project team visited New Moon to fill out their roster so they could try 20 man war. Loonies and crew returned to their alliance after doing their attacks. Mystery solved.