Please explain your feature request to the best of your abilities:

When a city belonging to a nation attacks or defends a capture site, let other member cities of the same nation assist by reducing HP and killing enemies. There could be an option to enable/disable assistance from members of the same nation for an attack or defense of the capture site.

We could go even further with an option to allow allies of the nation to participate.

There you go! Thank you in advance for reading my suggestion! Have a great day :)