战斗发生在当两支敌对的军队到达同一省份时。这是战争十分重要的一个组成部分。
三军
战斗中的两方军队将会各形成三路军队：
- 左翼
- 中军
- 右翼
每路军队通常将会分别对抗对手的同一路（左翼对左翼，右翼对右翼，中军对中军）。每路军队需要一个指挥官和下属于此路的士兵，为了拥有多于一路的军队，你需要多支不同的部队，尽管他们可以合并为同一支。
当一路军队没有与之对应的敌军（如己方左翼击败敌方左翼，且敌方左翼已经撤出战斗）可以攻击的时候，此路军队将会攻击地方另一路军队作为替代，并且将会得到一个30%的侧翼攻击加成，即多造成30%的士气和士兵杀伤。此时敌军的一路军队将会被己方两路军队攻击，将会受到更多的伤害。
战斗阶段
战斗分为三个阶段:
接触阶段: 所有的战斗都将开始于接触阶段。此阶段是弓箭手和轻骑兵（骆驼）及骑射手所擅长的阶段，但其中将只有一种兵种产生主要输出。
近战阶段: 经过若干天后，战斗将会转换为近战阶段，也就是重步兵和长枪兵及重骑兵擅长的阶段。需要注意的是各翼的战斗阶段不是必须统一的，各翼通常进入近战阶段的时间略有不同。
追击阶段: 当一翼的士气降低到25%时，此翼将会撤退，并且战斗将会转换到追击阶段，这也是轻骑兵非常擅长的阶段，同时重骑兵在这一阶段也有不错的表现。
撤退: 经过5天后，撤退的一翼将会脱离战斗。
兵种攻击与防御
影响士兵攻击或防御的公式是:
(基础单位属性) * (1 + 科技加成 + 地形加成 + 文化加成 + 战术加成) * (1 + 指挥官特性加成 + 战术克制加成)
单位属性统计表
基础单位属性取决于单位种类和战斗阶段:
| 攻击 | 防御 | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| 士气 | 占用人力 | 接触阶段 | 近战阶段 | 追击阶段 | 接触阶段 | 近战阶段 | 追击阶段 | ||
 |
轻步兵 (LI) | 2 | 0.7 | 1 | 1 | 2 | 2 | 1 | 2.5 |
 |
重步兵 (HI) | 4 | 3 | 0.25 | 6 | 1 | 3 | 4 | 4 |
 |
长枪兵 (P) | 6 | 2.5 | 0.1 | 4.5 | 0.2 | 4 | 4.5 | 2 |
 |
轻骑兵 (LC) | 4 | 3 | 2 | 3 | 6 | 4.5 | 3 | 4.5 |
 |
重骑兵 (HC) | 10 | 6 | 0.5 | 10 | 4 | 4 | 5 | 7.5 |
 |
弓箭手 (A) | 1 | 2 | 2 | 1 | 2 | 1.5 | 1.5 | 2.25 |
 |
骑射手 (ST) | 3 | 4 | 4 | 3 | 7 | 4 | 4 | 4.5 |
 |
战象 (ST) | 15 | 20 | 0.25 | 25 | 0.25 | 5 | 15 | 2 |
 |
骆驼骑兵 (ST) | 5 | 3 | 4 | 7 | 2 | 4.5 | 3 | 2.5 |
 |
船只 | - | 300 | - | - | - | - | - | - |
ST = 特殊士兵 (取决于宗教/文化)
军事科技
- 主条目：科技
科技加成 取决于军事科技级别: +0%/+10%/+15%/+20%/+25%/+30%,取决于现有的科技等级。
- 对于征召兵来说，即为该该征召兵隶属的伯爵领的科技等级。
- 对于征召封臣部队来说，即为附庸的平均科技等级。
- 对于常备军来说，即为首都的科技水平。
地形
地形决定了战斗所发生的环境。防御方通常会拥有优势，而地形起到了至关重要的作用。
下列地形会给予防御方地形加成:
- 丘陵
- 山地
- 丛林
下列两个省份之间的要素将会提供给攻击者一个地形惩罚。
- 河流:要查看两个省之间是否有河流(或海峡)，请选择其中一个省并查找图标
. - 主要河流:提供双倍跨河惩罚。
- 海峡
- 两栖登陆:几乎和海峡穿越一样。注:如果你控制一个伯爵领中的至少一个男爵领，你可以让你的船进入该伯爵领的港口。你的士兵将在没有两栖登陆惩罚的情况下将被卸载并与敌人战斗(但在战斗开始时只有上限50%的士气值)。
还有一些其他的地形类型不会给战斗的任何一方额外的奖励或惩罚，尽管它们会对补给限制产生负面影响（尤其是沙漠） - 损耗 打赢了很多战争.
 丘陵防御
|
山地防御
|
 跨河
|
 跨海峡
|
 两栖登陆
| |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 攻击 | 防御 | 攻击 | 防御 | 攻击 | 防御 | 攻击 | 防御 | 攻击 | 防御 | ||
|
轻步兵 | +10% | -10% | -10% | -15% | -15% | -15% | -15% | |||
|
重步兵 | +10% | -15% | -15% | -20% | -20% | -20% | -20% | |||
|
长枪兵 | +10% | +5% | +20% | +10% | -15% | -15% | -20% | -20% | -20% | -20% |
|
轻骑兵 | -12.5% | -12.5% | -17.2% | -17.2% | -17.5% | -17.5% | ||||
|
重骑兵 | -15% | -15% | -20% | -20% | -20% | -20% | ||||
|
弓箭手 | +20% | +15% | +30% | +20% | -10% | -5% | -15% | -10% | -10% | -15% |
|
骑射手 | +15% | +10% | +25% | +15% | -12.5% | -7.5% | -17.2% | -12.5% | -12.5% | -17.5% |
有限的侧翼
在战斗的近战阶段，以少打多的防御者可以利用地形瓶颈来抵消人数劣势。每个侧翼都可能有一个限制的方法，防止攻击者攻击的人数超过侧翼防守者的人数(指防守方一开始的人数)。[1]
发生这种情况的机会取决于主要省份的地形，在山区和丛林的几率最大。指挥官如果拥有高超的技能，战场地形大师
特质或者游戏大师
lifestyle trait特质也更有可能找到地形瓶颈。
当你在常规战斗中试图用数量压倒你的敌人时，这种技能的作用是有限的，当你使用
常备军来阻挡敌人时，这种技能的冷酷效果就会显现出来。对同时具有重步兵领袖Heavy infantry leader.png和战场大师特质的指挥官来说，如果能锁定在近战与有限的侧翼修正之中，这样的军队将能够给敌人造成可怕的损失，甚至可能赢得一场绝望的战斗。
Cultural Building
Cultural buildings can provide a cultural bonus to units trained in that holding. All cultural buildings have 4 levels, with each level providing an equivalent boost to some unit. The boost only applies to troops trained in the holding and to retinues (if the building is in the capital).
Combat Tactics
- 主条目：Combat tactics
Tactic bonus ranges from -150% to +420%. Note that these are so huge, they often swamp technology-based and terrain-based modifiers. Tactical affinity bonus ranges from -120% to +300%.
Commanders
- 主条目：Commander
You should assign a skilled commander to lead each flank (left, center, or right) that contains troops. You may assign yourself, your marshal, and direct subjects holding the minor title of "Commander". Hiring mercenaries or holy orders temporarily adds their leader and commanders to your pool.
Regiment commanders
You may also assign leaders to regiments, the sub-units within each flank:
- By clicking the regiment, if it belongs to you personally (e.g. demesne levies or retinues)
- By merging two armies
- By raising troops from a vassal whose marshal is idle (e.g. the Ecumenical Patriarch or a vassalized holy order with no counties)
Regiment leaders contribute to the battle alongside flank leaders, though the exact mechanics are unclear. Perhaps more importantly, if their flank's commander is killed during battle, a regiment leader can immediately take over and avoid leaving the flank in disarray.
Green commanders can be assigned to regiments in order to gain valuable combat experience without compromising the effectiveness of the army.
Battlefield Duels
Battlefield duels are personal battles between two commanders in which both participants’ stats are weighed against each other to determine the outcome, usually injury or death.
Damage
Every day, each soldier from a flank will make an attack on the enemy, causing damage based on its attack value. All damages for the flank are added together and sent to the flank that's being attacked.
The defense of the receiving units is computed, and 
casualties are inflicted as:
TROOP_KILL_FACTOR * (damage received / unit defense). TROOP_KILL_FACTOR is defined in defines.lua as 0.015.
In addition, each soldier who dies does 3 morale damage to the survivors (MORALELOSS_FACTOR).
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, morale will then be down by 1%. This means that deaths are a double penalty, both reducing the number of morale points a flank has, and reducing the number of soldiers.
Note that 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 20% or they're manually withdrawn from combat. Flight takes four days.
Prestige and piety
Each battle results in 1 prestige for every 100 losses of the losing army. A portion of this prestige goes to the characters involved:
- 50% goes to the owners of the units (divided among them)
- 25% goes to the main Commander of the centre
- 12.5% goes to each flank Commander
In addition, for every 1 prestige gained, characters will gain 1.5 piety if fighting religious enemies.
Strategies
See Combat mechanics thread and The Art of War, Playing in Europe for detailed insight on possible strategies.
Upgrade and specialize your holdings: combat tactics will, during combat, temporarily make some units incredibly powerful and others basically useless. For example, an army doing Swarm will temporarily have quadruple horse-archer damage and no damage from archers and light cavalry. So specialization is the name of the game.
Make it all your unique retinue unit: by using retinues alone in a flank, certain combat tactics that are particularly beneficial can be more easily selected as there are just one or two unit types in the whole flank; they also will benefit from the cultural unit bonus. Nomadic horde troops also benefit from this.
Flank composition: the Rule of Two: In order to maximize combat tactics efficiency, a flank should contain no more than two of the three major skirmish units (Horse Archers, Archers, Light Cavalry), and no more than two of the three major melee units (heavy infantry, pikes, and knights).
Bait enemies into attacking fortified positions: Ideally, your army is on a mountain, and across a major river from the attacker.
Avoid attacking across rivers and into hills: you'll get a moderate combat penalty, which could make a major difference. These penalties don't matter much if you outnumber the enemy by a lot, though.
Order early retreats when losing a battle. Simply order your army to move to another province, and it will begin retreating regardless of morale.
Scout and match. Send a tiny army to determine the strengths of your opponent's flanks. Then rearrange your main army to have two "holding" flanks, evenly matched against the opponent, and one "breakthrough" flank, with extra troops and your best offensive commander.
Single column concentrated attack: Light Infantry Army such as Tribal Army, and even Nomad Horde Troops may enjoy better morale preservation if they are concentrated in a single-flank. Once a flank is routed, it will not be able to deal damages. Thus, having all the army in a single flank ensure they stay in the fight until they break while the enemy army is broken piece by piece. This can help to fight against enemy in superior number (beware of Narrow flank, though enemy is more prone to attack you directly if they're superior in number). This can be concerted with scout battle to determine enemy weaker flank. Stack your corresponding flank and defeat that flank in priority, leaving them only two flanks left. Be careful however, as the two flanks left unattacked will gain a +25% flanking bonus that will boost their damage.