We have 12 players with a really good geographical spread so all human players will find at least one other human player not far away

These players are from a combination players using the Slitherine Forum and the Grogheads Computer gaming forum so I will be using both to keep in contact prior to the game start.
I am thinking of implementing a single house rule to ensure all players get a good run at the game and would appreciate feedback via a PM to me to see if other players agree..........
The proposal :
Human players can make co-operation treaties and non aggression pacts with other human players as well as agreeing to come to their military aid if they are attacked. However human players will not be allowed to join in offensive wars with other human players )...........................
My thinking is it will stop one player being ganged up on by several other human players who may have decided to parcel up their lands........(I know how nefarious wargamers are

I look forward to receiving any feedback (positive or negative) on this idea via PM.
Cheers
Ian