Starting a war
The aggressor may pick a Casus Belli when declaring war. This determines the wargoal, the options available in the subsequent peace deal, and their associated costs in warscore, aggressive expansion, and diplomatic power.
No Casus Belli
Declaring war without a Casus Belli will cost the attacker -2 drop in Stability and give additional Aggressive Expansion.
Sides in a war
A war consists of two opposing sides of one or more countries.
While wars can have many participants the country declaring the war and the country being declared upon will initially be considered the War Leaders.
War Leaders will negotiate on behalf of the entire side (including all allies and subject countries) to which they belong and a war will not end until the two War Leaders agree to a peace.
War Leadership in Imperator can also change. If a country with higher Max Manpower and more provinces than the existing leader joins, then this country will be considered the new War Leader and can call its own allies. When this is the case the War Declaration interface will warn that a country may take over the leadership of the war, and War Leadership can only be transferred once.
Sue for Peace
It is also not possible to separate peace as a junior participant of a war until 36 months have passed, as a junior participant is expected to be involved in order to help the war leader bring the war to completion, rather than using someone else's conflict to expand themselves.
After a peace has been signed a truce will be in power for between 5 and 15 years. The length of the Truce will be dependent on the severity of the peace agreement. During this time it will cost 5 Stability to declare war again against the same enemy.