Well there was a reason... Villages.
I'm not defending Nam by any means and what they did was 'wrong'. However, from their perspective you had a small ally that was beginning to come under fire from another, much stronger tribe ANY and were probably 'asking' for Nam to stack them from these attacks. Nam, who are gearing up for a big war wouldnt have wanted to spend all that def defending an 'ally' that would do a sub par job compared to their own members at defending those villages.
Personally, I think INT knew (or should have known) that this was going to happen and decided to side with Nam to last a little longer. They would have either had to face Nam at the end of the world (in the case of a Nam victory), merged into Nam (was never going to happen) or backstab Nam, while they were busy at war (most likely outcome).
In any case, boo hoo Nam are mean. INT i hope you enjoyed your time on the world, dont quit though make them work for the villages (morale is your friend)