How to start a war?

[Bibob]

So I am in control of America as the resistance and the academy is in control of brazil, they are currently fightinga war with the european union under the humanity first faction. I am in a rivalry with brazil and it says I fulfill all the conditions to declare war on Braszil. I want to do this because I want to send an army there to fight alien megafauna but I can't find the button for declaring war. Where is the button for war declaration situated?

[Akameka]

it is not a button, you have to send an agent in your country to vote a new policy, and use it to start a war with the country.

[Lovecraft]

You also have to declare them a rival first (button in top right corner of country window).