Pro-tip: never simply declare war on someone. Let one of the other factions pay you to go to war
No 'declare war' button necessary.
I define this a major oversight and a painfully cheap exploit that, imo, needs to be addressed.
I'm completely fine with the AI coming to you and offering you to go to war with whom THEY want you to go to war with. I'm also completely fine with you going to them and paying them to start a war with someone you plan to go to war with.
However, never should you be able to pay yourself to go to war with your intended target. The diplomacy and AI just isn't near good enough for that level of intrigue. For such a mechanic to exist properly, the AI would have to be able to judge how willing you are to declare war on another faction and adjust their offer accordingly.