Wars allow States to battle others with armies in order to inflict penalties or force diplomatic actions onto another if won, such as paying tributes, joining Nations as vassals, losing Victory Points, or conceding territories.
How to start a war
This can be done by selecting a foreign territory of the target on the world map, and from its owner's character profile clicking the "diplomatic actions" button and selecting "declare war".
The war declaration menu then allows you to select a casus-belli and to preview the initial belligerents on both sides, the starting war morale for each, the war type effects and objectives, and to select optional co-belligerents you wish to call-to-arms in this war.
Main war mechanics
Wars always involve 2 main belligerents: a main aggressor who started the war, and a main defender as its target.
Each one can have one or more co-belligerents participating as allies in the conflict.
Each war is of a specific type associated to the casus-belli used to declare it. The war type defines the war objectives of each side, the rules to bring co-belligerents, the available military actions to each side, and modifiers of the initial war morale and its passive evolution.
Co-belligerents are the allied participants in a war who help their main belligerent win its objectives.
End of hostilities
When hostilities end between 2 characters (whether it's because the war itself ends, or because a co-belligerent is eliminated), all the territories that belong to one and are occupied by the other are automatically returned to the original owner. All troops that are standing on the other's territories are also auto-repatriated to the nearest regional capital.
A war ends when one of the main belligerent is eliminated, which can happen if the main belligerent players sign a peace treaty manually or automatically if one reaches a war morale of 0 (or capitulates).
When a war is ended automatically, the predetermined war effect of the winning side apply. These often involve the auto-signing of a treaty with terms for the losing side. Each war type comes with different predetermined effects: humiliation of rivals will remove Victory Points, annexation wars will transfer a territory to the winner, extortion wars will create a treaty taxing the loser...
No conditions for casus-belli. Allows only to raid the enemy. Can only end by manual peace-treaty or by the aggressor being defeated.
No conditions for casus-belli. A war to force the defender to pay tributes to the aggressor.
Requires a broken treaty casus-belli. Similar to the extortion war, but with a much stronger initial morale for the aggressor.
Requires a fabricated claim casus-belli. If the war is won, the target territory is transferred to the aggressor, but it gives the defender a 'liberation' casus-belli for that same territory.
Requires a liberation casus-belli. Similar to the annexation war but with a much stronger initial morale for the aggressor.
A war with the objective of completely subjugating another State, forcing him to join the Nation of the aggressor on top of paying tributes.