[Merry Jolkar]

I think the simplest solution would be to revert back to the standard rules if you break pact. i.e., you fight. If there are other races at that location, you fight in the order you would normally fight in, had there been no pact.
If, however, you ally with the enemy on the same turn you break pact with your former ally, you switch sides and fight on the side of your new ally against your old ally.