War is waged between two or more factions in order to capture, sack and raze enemy provinces and settlements, and to achieve military dominance. You can use the diplomacy screen to declare war on other factions. Using your army to attack another faction's settlement or army, will result in an automatic declaration of war on them. However, the game will warn you before this happens.
A declaration of war on a faction may also result in war with that faction’s allies or defensive allies, as they are called in to assist. If you choose to declare/start a war, then you can call your military allies to assist you (they can refuse, but their reliability rating will decline). If another faction declares war on you, you may call on both your military allies and defensive allies. A state of war will persist until one side is destroyed, or a peace deal is agreed through diplomacy. Breaking a peace deal soon after making it, will result in the breaking faction's trustworthiness rating to decline.
The following actions will NOT result in war, but may worsen diplomatic relations between two factions (they will like each other less and be more likely to declare war).
- Aggressive actions by heroes against another faction's settlement or army.
- Raiding other faction's lands.
- Moving your army onto another faction's land (trespassing) without having a military access agreement.
- It goes without saying that you should really pick your engagements carefully. Always analyze the enemy's alliances (if any) and current strength. There are no sentiments in grand politics, so there's no shame in pouncing on a wounded faction that just scraped together money for a peace settlement with another foe.
- Especially if it's a faction that's been a thorn in your side and demonstrated remarkable resilience in the past.