War

War can be declared by one House against another for the sake of justice or conquest. Wars can end in one of four ways: if one side is victorious and conquers the other, if one side surrenders to the other, if one side proposes peace and the other accepts, and if neither side attacks the other for a long enough period of time and the war is allowed to lapse on its own.

Codex
"When relationships between two leading houses break down, one side can declare war on the other. Simply declaring war is a dishonorable act. A war becomes justified if it is declared on the basis of avenging a crime, or to champion a better claim to lead the defending faction.

When a great house is at war, all the factions in its league are considered at war with the enemy. Some political actions are not possible between houses whose respective domains are at war.

While at war, any leader on both sides can attack and conquer systems owned by the opponent. When a system is attacked in war, the defense is always the responsibility of the leader of its faction. If a system is conquered, the victor may choose to replace its existing ruling house by with a more trusted vassal. Annexing systems that one has no claim to is also considered dishonorable.

The war ends when one side or the other is wiped out, or when they mutually agree to peace."