A fair point. I've left it particularly vague to see how people interpret the question and react. You would assume in this situation that you would have noticed a military build up in some of the ally's territories but not noticed them moving toward any known hostile border - then suddenly they declare war. Being the skeptic I am of allegiances with people I don't know well (i.e, real life buddies/friends), I would probably have a small garrison or two along that border that could double as a slowing force and scout.