You state a reason for war on the user above! The user below states why your casus belli is invalid then declares their own reason for war on that person!
Example:
User 1: I declare war because the treaty of 1792 guarantees our right to choose succession based on that of who can throw chicken giblets the furthest and he didn't throw!
Me: The pope declared that treaty invalid in 1793!
I declare war because User 1 stole my chickens!