Combat in Imperator occurs when at least two armies of states currently at war with each other are in the same city. The army first in the city is considered the defender, unless the other side has control over fortifications in the province.
On each side, more then 1 army can partake in combat. This is easiest achieved by attaching armies to a leading army. Armies can be attached to own armies and also to armies of allies.
In Imperator: Rome, the battle field map is divided into squares. The squares are grouped into 4 rows, 2 rows for each side, each row containing ? squares. One cohort fills one square.
There is a Primary Frontline and a Secondary Frontline. The First Frontline will enter battle first, damaging the opposing side until its morale breaks or it suffers enough damage to be eliminated. The Secondary Frontline will then begin to move forward to become the new front. On the sides the units designated as Flank units will be deployed; these will first fight and kill the opposing flank if there is one, and then start targeting the center if they can.
In the army interface, you can select which unit type you want to be prioritized for First Frontline, Second Frontline and Flank. The preselected choices will depend on your Military Traditions but they can be changed freely by the player or the AI for each army. The size of the flank can also be set, either 2 cohorts, 5 cohorts or 10 cohorts.
Additionally some unit types deal or take more morale damage or more physical damage, making them more or less suited for each role. In most cases this means that there is effectively a first skirmish phase where the Primary Frontline of Archers or Light Infantry try to do as much damage as possible to the other side before their morale breaks and they retreat. They are then followed by the units of the Second Frontline, potentially composed of heavier units such as Heavy Infantry or/and Elephants.
The best units to deploy on the flanks are usually ones with a high maneuver value, such as Horse Archers, that would allow them to deal damage far into the center once they have defeated the opposing flank. However, it may be better to prioritize countering the opposing flank.
When there are not enough of the preferred unit type for a role the game will fill out with units in order of how high their build cost is.
Each day, each cohort (in the first battle row) will fire on one target cohort (in the first battle row) either directly in front of them, or towards the side, if they have a high enough flanking ability. Casualties and morale damage inflicted on the opposing unit are based on several factors, including leader, terrain, unit types, tactics, research and a random dice roll.
Units with depleted Morale (at some small value ~0.10) disengage immediately and stay in reserves until the end of the fight. Next day after they disengage, other units or reserves take their place. The next day after the last available unit from one of the sides disengages, combat ends and the losing army goes into a shattered retreat. Unlike e.g. CK2, there is no pursuit phase with one side running down and killing broken enemy.
After 5 days of combat have passed, it becomes possible to order a retreat. The battle is considered lost with corresponding warscore and general popularity losses. But since there is no pursuit phase or additional casualties to the retreating side, it can still be better than continuing a very disadvantageous battle to the bitter end.
Each army in the same combat can be ordered a manual retreat separately by selecting it from the outliner. Cohorts of remaining armies continue fighting and deploying reserves to fill in abandoned positions. This means, that in case you grouped your skirmishers into an own army, you could have them retreat on their own after a few days without waiting for them to become completely ineffective, and have them start recovering morale and manpower while the battle continues.
Note: this formula is original research and as such could contain errors. If you see in-game results not matching it's predictions, please correct. So far the only discrepancies are off-by-one, probably due to rounding down intermediate results in calculations.
pips = Dice Roll + Leader + Terrain
Base Damage = 0.08 + 0.02 × pips
Damage = Base Damage × (1 + Discipline) × (1 + terrain Bonus) × (1 + vs Unit type) × (1 + Tactics bonus) × (1 + Offense vs Defense) × (1 + vs Experience)
Kills = Damage × Men * (1 + vs Casualties from Tactics) × 0.2 (Daily multiplier) × (1 + vs Unit type)
Morale damage = Damage * (Men ÷ 1000) × (current Morale ÷ 2) × 1.5 (Daily multiplier) × (1 + vs Unit type)
One notable difference from EU4 is that current, not maximum morale is used to determine morale damage, so worn down units inflict much less morale damage on the enemy. Also units that are flanking or in reserve do not suffer any daily morale hits.
- Main article: Terrain types
Crossing a river or attacking into Hills, March and Forests gives -1 to the attacker, while crossing a strait, doing a naval landing or attacking into Mountains gives -2.
Leaders impact combat as well, with every 2 levels of martial skill difference giving a +1 bonus to the better general.
A random value between 1 and 6 is rolled every 5 days for each side.
Total number from adding up the dice with the combat modifiers from terrain and leaders determines the base damage.
On average you can expect to roll (1+6)/2 = 3.5 and with equal generals and no terrain penalties have a base damage of 0.8 + 0.2*3.5 = 0.15. In the absence of other modifiers this corresponds to 1000-man cohort with 3.0 morale killing 0.15*1000*0.2 = 30 men and inflicting 0.15*1*(3/2)*1.5 = 0.33 morale damage per day.
Each pip increases or decreases base damage by 13% of this expected average. The worst possible base damage is probably 0.04 (one quarter of average), having -3 from terrain and rolling a 1 with an equal or worse general. The best possible is probably 0.34 (more than double the average and 8.5 times the worst), rolling a 6 with 14-skill general vs no general.
Various sources of discipline such as tech, trade goods and blessings are additive. In a strange exception to this rule, combat tooltip seems to calculate +10% improved discipline from Personal Loyalty to a general twice: both multiplicative and additive. E.g. with +15% country Light Cavalry discipline, a loyal cohort shows +26.5%. The difference of 1.5% is small enough that it's difficult to tell if the tooltip is wrong, or if it's actually calculated this way.
Every unit accumulates experience in battle, which then reduces all incoming damage at a rate of 0.3% of damage reduction per 1% of experience. So a unit with 100% experience will have a 30% damage reduction. Accumulated experience decreases over time again.
In summary, the variables that impact the combat the most are:
- For morale damage - current morale. A unit at full morale does 5x morale damage of a unit that is on low maintenance.
- With small armies - flanking/maneuver. Flanking with 5 horse archers on each side makes enemy side units take 6x damage, while horse archers themselves stay at full morale and manpower.
- General skill and terrain. A 5 pip advantage (e.g. 10 skill general vs 1 skill general) gives +100% base damage.
- Heavily countering unit types. E.g. Light Cavalry vs Heavy Infantry does only 50% normal damage.
- Low manpower. A battered non-consolidated 500-man cohort will only do 50% of the damage of a full 1000-man cohort, while still taking a full space on the front line.
For each army, exactly one tactic is chosen out of battle. Each tactic is strong (giving a bonus to damage) against two other tactics, and poor (giving a penalty to damage) against two more. The base effectiveness of a tactic is determined by the army's composition, with each matching unit type giving a bonus to effectiveness. A given tactic will be useless if the army has no cohorts that give a bonus to it.
The first five tactics are available to all countries, while the others are unlocked by military traditions.
|Tactic||Unit effectiveness||Against other tactics||Casualties||Description|
|Bottleneck||Against a massed charge, nothing performs better than a solid defensive line. However, if the enemy is clever enough to pick us off one by one, we may encounter problems.|
|Cavalry Skirmish||−10%||Ordering cavalry to harass and skirmish, rather than remain in formation, can often be used as a tool to deny an entire flank to hostile troops.
Requires Greek war tradition "The Companion Cavalry", North African war tradition "Wild Charge", or Persian war tradition "Cavalry Skirmish".
|Deception||A staggered assault can wear down an enemy's resolve faster than one might imagine, and allows us to respond to mobile threats with great ease. The greatest weakness of this tactic stems from our vulnerability to skirmishing behavior.|
|Envelopment||Drawing forth an enemy counterattack, and then plunging into the side of their exposed formation can cause massive losses. Against an enemy who can quickly martial their men to multiple fronts however, increases the risk of this maneuver.|
|Hit-and-Run||−10%||In the face of an overwhelming enemy an asymmetric approach can often be more successful than a head-on one. Ambushes, raids, and hit-and-run style tactics were common in ancient warfare, especially in Gaul, Germania, and Iberia
Requires Barbarian war tradition "Ambush".
|Padma Vyuha||−10%||A highly complex defensive formation, the labyrinthine appearance of the Padma Vyuha is designed to confuse and misdirect foes while defending more vulnerable friendly troops at the core.
Requires Indian war tradition "Padma Vyuha".
|Phalanx||−10%||The Phalanx originated as a highly defensive method of formation fighting, used primarily by Greek city-states. It was further developed by the Macedonian military, who built their armies around a heavily armored Phalanx formation.
Requires Greek war traditions, or Levantine and Arabian war tradition "Greek Warfare".
|Shock Action||+10%||Sometimes, caution must be thrown to the wind - few foes can stand against a massed charge, though we must be wary of those that can field a staunch defense.
This is the default tactic for any new unit.
|Skirmishing||−25%||If the enemy exposes a series of flanks for us to harry, this maneuver will surely pay off. We should not employ this tactic against stalwart offensive lines, however.|
|Triplex Acies||Like the Hellenistic Phalanx the Roman tactic formation known as the Triplex Acies, or triple lines, is inspired by the Phalanx of the Greek City states. Where the Macedonian or Hellenistic Phalanx has gone for cohesion the Roman Formation instead emphasized flexibility.
Requires Latin war tradition "Triplex Acies".