You should buy systems with influence, long before the other player declares war. And as fast and as much systems as you can in same turn.

This mechanic is also dubbed "peaceful conversion" and that should give you an idea. If you always could convert systems, there would be no counter at all.

If your opponent declares a war, then he just gave you an excuse to take that system from him by force.

Allright so I guess there is no way (either direct option or mod) to disable this.

I see what you mean but I maintain that I think it ruins a little bit the dynamic. There is no point in doing peaceful conversion against human player because just before you can take the planet they will declare war on you. So instead of having invested your time on influence you should have started to invest it on military directly.

Also even in "reality" you could imagine two countries at war and on region is rebelling at some point to join the other nation. So even under this perspective it makes sense to have influence conversion even at war.

I think it is a pity because I find this mechanic of gameplay super interesting and original (I usually played civilization). But it is my view on this of course =)

I guess I have my answer now anyway =)

Thank you very much.

Isn't there an option for the empire with higher war momentum to try and force a truce? Then after the truce period when you return to cold war I bet you can click the buyout button faster than your friends can declare war again.