This article is accurate for the latest versions of EU3, Napoleon’s Ambition, In Nomine, Heir to the Throne and Divine Wind.

A warning is a diplomatic option that promises the defense of one country by another, whether or not they are allied. It allows the warner to join in a war to defend a nation being attacked by the warned. In the event that the warned nation attacks regardless of the warning, the nation giving the warning receives a Call to Arms and then has the option of joining the war. In Divine Wind warnings only call someone to arms if they neighbour the warned nation. This helps prevent the odd alliance chains seen in older versions.