Waging war is one of the primary ways to attain territory. Warfare is centered around a few concepts, namely levies, combat, and sieges.
In CK2, wars are fought over Casus Belli. The CB you use has a direct impact on how you wage your war. Your goal is to raise the warscore to 100%, which will force your opponent to accept your demands. You opponent may offer to surrender before that, but at 100% he MUST surrender. If you're on the defensive, or need to bail out of the war, you can also seek White Peace when the war has gone on for a bit and the warscore is still near even.
Warscore can be gained in four ways:
- Winning battles against enemy armies.
- Sieging down and occupying enemy holdings.
- In some CBs, the warscore will tick up or down if the targeted province is fully in the attacker's or defender's hands.
- Holding prisoners. If the leader of a war is captured, warscore becomes 100% in his enemy's favour. Captured close relatives of the war leader also affect the war score.
Warscore gains from victories in battle or from occupying holdings are larger if they're against the war leader as opposed to his allies.
Every day in each flank each soldier will take a swing at the enemy. That swing does damage equal to the unit's attack strength, multiplied by any modifiers, multiplied by 0.015. All damage for the flank is then added together and sent to the flank that's being attacked.
The defending flank will then have the damage distributed according to its unit composition. So if 20% of the flank is archers, then the archers will receive 20% of the damage. That damage is divided by the defense strength, multiplied by any modifiers, of all units of that type. This value is then rounded down unless it is below 1, and as many soldiers will die.
Each solder who dies does 6 morale damage to the survivors. The survivors have morale points equal to the sum of their morale values, so if they take 60 morale damage but the survivors have 6000 morale points then morale will be down by 1%. Note that this means that deaths are a double penalty, both reducing the number of morale points a flank has, and reducing the number of soldiers. Apart from the ratings of troop types changing all phases are identical.
The calculations happen simultaneously every day for every flank involved in the battle, so all values will be calculated based on the number of troops present before anyone dies. Do note that in the Pursue phase the fleeing flank will not make any attacks. Combat ends once all flanks on one side are either dead or have fled. Flanks attempt to flee once their morale hits 25% or they're manually withdrawn from combat. Flight takes four days.
 Character Modifiers
Every army can have up to three generals: fighting in the center and the two flanks. The skill of the general adds directly to his troops' combat effectiveness and allows the general to choose more advanced tactics - which will also increase his troops' effectiveness. In addition, any adult character who has been educated in the martial attribute is given certain combat bonuses - the specific bonuses of each martially-trained character are visible underneath their list of traits on their Character screen.
Do note that generals can only be assigned under Limited crown authority or higher. Below is a table of all leadership traits.
Terrain defines the surroundings where battles are fought. The defenders usually have the advantage and terrain can play a vital role in a fight.
There are two types of terrain which give bonuses/penalties to battle in:
Hills provide the defenders the following bonuses: (Defense: Archers +20%, Pikemen +10%, Horse Archers +15%. Offense: Archers +15%, Pikemen +5%, Horse Archers +10%).
Mountains provide the defenders the following bonuses: (Defense: Archers +30%, Light Infantry +10%, Pikemen +20%, Heavy Infantry +10%, Horse Archers +25%. Offense: Archers +20%, Pikemen +10%, Horse Archers +15%)
There are also several other terrain types which don't give a bonus or penalty to either side of a battle, though they can negatively affect the supply limit (especially true for deserts) - attrition has won many wars.
 Rivers and Straits
These two terrain features lie between provinces and give a penalty to the attacker. Avoid attacking over these features unless you have overwhelming numbers. To see if there is a river or strait between two provinces, select one of them and look for the icon .
Here's what it looks like in-game:
Attacking across a river provides the attacker with the following modifiers: (Defense: Archers -10%, Light Infantry -10%, Pikemen -15%, Heavy Infantry -15%, Light Cavalry -12.5%, Knights -15%, Horse Archers -12.5%. Offense: Archers -5%, Light Infantry -10%, Pikemen -15%, Heavy Infantry -15%, Light Cavalry -12.5%, Knights -15%, Horse Archers -7.5%)
Attacking across a major river provides the attacker with twice the penalties as attacking across a normal river (for instance: -20% archer defense instead of -10% archer defense)
Attacking across a strait provides the attacker with the following modifiers: (Defense: Archers -15%, Light Infantry -15%, Pikemen -20%, Heavy Infantry -20%, Light Cavalry -17.2%, Knights -20%, Horse Archers -17.2%. Offense: Archers -10%, Light Infantry -15%, Pikemen -20%, Heavy Infantry -20%, Light Cavalry -17.2%, Knights -20%, Horse Archers -12.5%)
 Amphibious Landing
You can send troops that are loaded on a fleet to attack an enemy in a coastal province. However, this will incur a significant penalty. To do it, select the fleet, then select the army within it. You can tell that army to move to any province on the coast of the body of water the fleet is in.
Note: if you control at least one holding in a county, you can have your ship land in the county. Your soldiers will be off-loaded and fight any enemies there without the amphibious landing modifier.
An amphibious landing provides the attacker with the following modifiers: (Defense: Archers -15%, Light Infantry -15%, Pikemen -20%, Heavy Infantry -20%, Light Cavalry -17.5%, Knights -20%, Horse Archers -17.5%. Offense: Archers -10%, Light Infantry -15%, Pikemen -20%, Heavy Infantry -20%, Light Cavalry -17.5%, Knights -20%, Horse Archers -12.5%)
They are almost identical to a strait crossing except some modifiers are -17.5% rather than -17.2%.
 Game Difficulty
It is possible to set game difficulty before start. It ranges from very easy to very hard and morale of both your levies and the AI is affected.
|Army Morale||AI Army Morale||AI Reinforcement Rate|
All holdings have a basic garrison, which increases in size if certain improvements are constructed. Holdings also have a levy which can be raised. When sieging, you will need to have more troops in the county than the defenders have in their garrison plus levy. If the attackers do not have enough men, no progress will ever be made on the siege. Note that a liege can raise his vassals' troops and the vassals' garrisons will show their levy strength as being at full manpower. However, a character who raises the troops from his (or her) own demesne will have those raised levies taken out of the garrison. This means that it is much easier to siege the holdings of a liege who has raised his demesne troops than it is to siege his vassals.
If the attackers do have enough troops, every 12 days a tick will count and progress will be made on the siege. The amount of progress that is made per tick depends on the fortification level of the holding and how severely the attackers outnumber the defenders.
Eventually, the attackers will be able to assault the defending garrison. The amount of time it takes for this option to become available depends on the holding's fortification level and how much the attackers outnumber the defenders. If an assault is ordered, the attackers will start losing large quantities of troops while accelerating the siege progress substantially.
Whether slowly or quickly, once a siege reaches 100% progress, it will be occupied by the attacker and warscore will be awarded.
When assaulting, it is recommended to have 10x the number of troops involved in the attack as the defenders have in their garrison. This will minimize the attacker's casualties, as the defenders are overrun and unable to maintain any level of adequate defense. Assaulting is based on the skirmish value of the unit, making archers rather good at assaults.
Mercenaries are troops that can be hired for a price. They can be hired from the Military screen on the Mercenaries Tab. Note that not all regions have mercenaries available, and those that do can have different quantities available. In addition to the initial hiring fee, they have upkeep fees which are much higher than any of your own troops'. Be sure that you have plenty of money to pay them. Mercenaries who do not get paid will at best disband themselves, and at worst will defect to the enemy or declare war on you (and this can even happen in the middle of a battle). Also, when hiring mercenaries, the full force of their unit may not be available if their previous employer disbanded them while they were not at full force. They will gradually replenish their manpower over time. It is easy to mistakenly hire a mercenary band and then realize that you have only hired a few hundred troops rather than the few thousand that you expected - pay careful attention to their manpower on the mercenary screen to avoid this mistake. The cost of hiring a troop does not vary based on how many units they currently possess, but the cost of maintenance does decrease as the mercenaries take casualties. For this reason, and because mercenaries can replenish casualties while on the field (standard levies do not replenish on the field), it is beneficial to put mercenaries wherever casualties are expected to be highest.
The monthly upkeep of a company depends on that unit's current size. A thrifty and unscrupulous ruler might deliberately waste half of a company in a futile assault in order to save money.
Each mercenary company has a predefined capital province, and may also have predefined cultural or religious requirements. You may only hire a company if your capital is within a certain distance of the company's capital and if you meet the religious and cultural requirements.
Mercenary troops can only be hired by one character at a time - therefore it is very possible that, at any given time, all the mercenaries which are within your price point may be unavailable because they have been hired out by other realms. If your war strategy depends on the hiring of mercenaries, ensure that you check mercenary availability before declaring war.
 Holy Orders
Holy Orders are similar to mercenaries in that they can be hired and are independent of other realms. Holy Orders cost Piety to hire instead of gold. During Crusades/Jihads, as well as when defending against infidels, they have free upkeep. During offensive wars, however, they have a substantial gold upkeep cost.
The most important fact about holy orders is that they will not fight enemies of the same faith as them, so make sure you point them in the direction of heretics and infidels!
 The Home Front
If you are in an offensive war (except Crusades) and raise your vassals' levies, you will gradually incur a stacking opinion penalty, -1 for every 61 days (approximately two months). This penalty will increase as long as your vassals' levies are raised, and will slowly decrease at the same rate it increases. So if you're at war for 4 years, your vassals will end up with a -20 opinion of you which takes 4 years to wear off. If you're a long-reigning ruler with high diplomacy and good vassal relations, this is merely annoying. If you're barely keeping a lid on the dissent as it is, this can be extremely problematic. You can avoid this by calling as few of your vassals' troops as possible or by ending the war as quickly as possible.
Additionally, if your personal levies and retinue get depleted, you could have issues with factions, as they measure the strength of their members against the number of troops you can raise. If half your personal troops are killed off, the power of those factions will increase dramatically, making them more likely to make demands or rebel. This is potentially compounded by more members joining the factions as their opinions of you drop due to the raised levies. Losing many troops is, however, one way to trick a faction - which would normally be too weak - into revolting, thereby allowing you to crush a relatively weak faction and revoke their lands at a mildly convenient time, rather than risking them continuing to gain power and strength and revolting at a truly inconvenient junction. This assumes that you have the wealth to buy mercenaries or that the faction is weak enough to not steamroll assaults through your holdings - you will probably need a bit of time to replenish all those troops you lost, after all.