Please look at this screenshot first. I manually put a number to respective Civilizations for better explanation.

At my previous gameplay, I made an Alliance with Civilization-2, just to find out it was fighting a war against Civlization-3. But apparently Civilization-3 was a liege lord of Civilization-1 and Civilization 4. So I decided to join the war to help my faithful ally. First I made some demand to increase War score. After accumulating more than 80 War score I was able to declare war on Civilization-3. However, soon after that, Civilization-3 lost the war and become a vassal of Civilization-2. This made all of the remaining previous vassals also vassals of Civilization-2. Just like the screenshot below.

The obvious problem was that I was uninformed of this important event in any way possible, and all of a sudden my war against Civlization-3 was canceled without further notice. I realized that only after reading pop-ups, which said that I had to declare war to enter this territory(of Civilization-4).

Hence, I would like to suggest a new game mechanics to warfare. A new feature, which enable you to fight alongside other attackers. Whether you join the on-going war of your Allies by their Demand(request) or declare a war upon an enemy, which happen to be in the middle of war with other Civilizations, you and other war participant should immediately form a coalition.

In diplomacy screen you would see members of the coalition. In this screen there would be War Score:

Respective War Score: War Score of each member. Only affected by the actions of the member itself, not by allies' actions. Or simply understood as contribution to the War.

Total War Score: sums of War Score of each member. If Total War Score reach certain level, the coalition could propose Force Surrender to end it all. Maximum Total War Score is scalable to the size of the coalition. +100 per each member. Theoretically, one member could earn Respective War Score up to Maximum Total War Score if the other members achieved 0.

a graphic example:

*each colour represent Respective War Score. Together, they make up for Total War Score.

Once peace term is proposed, you would be able to either accept the peace term or refuse. If respective members of the coalition choose different response, then those who signed peace term would be ejected from this conflict. Those who decided to prolong the war, should remain in the war. (so that similar peace terms like the one between new Bolshivik government of Russia and Central Powers from WWI can be revived/re-written in Humankind)

In surrender Terms, only your Respective War Score can be used to demand something. Only Territories/Outposts captured and controlled by you can be demanded for balance purpose. (to prevent you from obtaining things you did not contribute to). Even when one of your coalition member make the enemy one of its vassals, you would still be able to obtain Territories/Outposts or Reparation, as long as you have enough Respective War Score to do so.

How do you like this idea?

And last but not least, regardless of the any viability of this proposal, at least Player MUST be notified if the enemy surrenders and becomes a vassal of another Civilization!