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
In Ymir, war is a diplomatic action that requires to select a casus-belli to be declared and to pay some Politics Points.
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
UI of an ongoing war
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
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.
War ending
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 if the player 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...
War types
Raiding campaign
No conditions for casus-belli. Allows only to raid the enemy. This type of war is special because it can only end by a manual peace-treaty agreed by both parties, or by the aggressor being defeated: the defender cannot capitulate and the attacker cannot auto-win.
Punishment war
Requires a broken treaty casus-belli. Similar to the extortion war, but with a much stronger initial morale for the aggressor.
Annexation war
The annexation war is the war type that allows one state to steal territories from another state: if victorious, the territory itself is transferred to the winner as direct ownership.
Liberation war
Requires a liberation casus-belli. Similar to the annexation war but with a much stronger initial morale for the aggressor.
Humiliation war
Only available against a rival, if won it removes 50% of its Victory Points. If lost, the aggressor loses 50% of his Victory Points.
Subjugation war
A war with the objective of completely subjugating another State, forcing him to join the Nation of the aggressor on top of paying tributes.