1.1 Combat occurs when a unit with an attack value greater than 0 attempts to enter a square occupied by a unit of another, non-allied, civilization.
1.2 A battle results in the destruction of one unit or the other. If the primary defender is destroyed, all other defenders (stacked) in the same square are also destroyed unless:
The primary defender is the unit with the greatest modified defense value at the moment of the attack.
1.3 Units involved in combat usually sustain damage. Damage is shown in the unit's strength bar by color and percentage.
Green: 2/3 to full strength
Yellow: 1/3 to 2/3 strength
Red: less than 1/3 strength
1.4 Except for air units, damaged units have reduced movement points approximately proportional to the damage. However, land units will never have less than 1 movement, and sea units will never have less than 2. The proportion of damage is rounded to the nearest integer, but never less than the minimum for that unit.
CMP = CHP/HP * MP + 0.2
CMP...current movement points
CHP...current hit points
HP...maximum hit points (from rules.txt)
MP...maximum movement points (from rules.txt)
1.5 Ground units can only attack other ground units. Exceptions: Helicopters can be attacked by ground units if they occupy a land square. A sea or air unit may be attacked if it is the primary defender in a city or airbase.
1.6 Air units can attack only ground or sea units. Air units with the "Can attack air units" flag (fighters) can also attack other air units. Air units attacking ground units ignore city walls.
1.7 Sea units can attack other sea units or any unit occupying a sea square or land square adjacent to a sea square, including cities. This includes helicopters, land units, or any air units caught on the ground in a city or airbase. Exceptions: Airborne bombers cannot be attacked by sea units. Units with the "Submarine advantages/disadvantages" (submarines, thus) cannot attack land units. When a sea unit attacks a land unit, it is called shore bombardment.
1.8 Special domain 3 units can be attacked by any other unit, regardless of flags. Domain 3 units can attack any other unit, and they ignore city walls, SAMs, and coastal fortresses.
Units have four basic factors figured into the calculation of a combat result: attack strength, defense strength, hit points, and firepower.
2.1 Attack strength is the likelihood of inflicting damage when attacking an opponent. Units with 0 attack strength cannot initiate combat.
2.2 Defense strength represents the ability of a unit to defend itself when attacked; It is the likelihood that damage will be inflicted on an attacking unit.
2.3 Hit points indicate how much damage a unit can withstand before it is destroyed. The true number of hit points is the hp value x10. A 2hp unit thus has 20 hit points.
2.4 Firepower indicates how much damage a unit can inflict in a round of combat. A successful round reduces the opponent's total hit points by the value of the unit's firepower.
Many factors affect the four basic Unit Combat Factors, depending upon the circumstance of the battle. All factors that apply as percentages are multiplied together.
3.1 Veteran units receive a +50% bonus for both attack and defense strengths
3.2a Fortified land units receive a +50% bonus for defense strength. This bonus is superceded by fortress improvement (3.2b) and city walls (3.4). It can be used in combination with a SAM battery (3.5) or a coastal fortress (3.6).
Notes:
This bonus applies only on the next turn, after the unit appears with the dirt entrenchment, not while in the process of fortifying.
It has been suggested, but not tested, that city defenders in multiplayer games do gain the bonus immediately if given the order to fortify with the 'f' key, but not the menu. (MdS suspects this is false in single player.)
Air units caught fortified in an air base do not enjoy this bonus. Fighters in unwalled cities may, however, gain the scrambling defense bonus, as if being attacked by a bomber - even if the attacker is a land or sea unit. (This point is still in contention, see posts 15 thru 17 and post 19, below.)
3.2b The fortified bonus increases to +100% for a land unit occupying a fortress improvement, whether given the order to fortify or not.
3.3 Terrain increases defense strength of all units by:
x150% if on forest, jungle, or swamp
x200% if on hills
x300% if on mountains
+50% if on a river, additively enhancing any other terrain bonus. For example, a rivered hill gives a x250% bonus, a (200 + 50)% multiplier.
3.4 City walls triple the defense value of city defenders against attacks by land units only. This bonus can apply only to land units.
3.5 SAM batteries double the defense value of city defenders against attacks by air units. This bonus applies to any unit defending a city except a scrambling fighter, which receives a separate defense bonus.
3.6 Coastal fortresses double the defense value of city defenders against attacks by naval units. This bonus applies to any unit defending a city.
3.7 Some flags (in rules.txt) can alter combat calculations as follows:
Negates city walls (howitzer): Self explanatory.
x2 on defense versus horse (pikemen):: This is actually a limited bonus, +50% if the attacker is a land unit with move=2, hp=1, and fp=1.
Can attack air units (fighter):: Self explanatory. A fighter stationed in a city that is attacked by a bomber or helicopter scrambles, gaining a +300% defense bonus. A fighter gains a +100% defense bonus when attacked by another fighter. However, a fighter cannot benefit from a SAM adjustment. Any helicopter attacked by a fighter suffers a -50% defense adjustment and has its firepower reduced to 1.
x2 on defense versus air (AEGIS):: An AEGIS cruiser gains a +200% bonus when attacked by an air unit, +400% if the attacker is a missile (destroyed after attacking).
3.8 Partisans gain an attack advantage against non-combat units (any with attack value = 0). Their attack value is increased eightfold (8x).
3.9 Shore bombardment: When a naval unit attacks a land unit, both units have their firepower reduced to 1. A sea unit's firepower is also reduced to 1 when it is caught in port (or on a land square) by a land or air unit; The attacking unit's firepower is doubled in this case.
3.10 SDI defenses thwart any nuclear attack (except those set by spies) within 3 squares of the city in which it is built.
3.11 Nuclear missiles have a special attack value of 99. Unless thwarted by an SDI, this sidesteps any combat result calculations. If a missile attacks a target, a strike results. This kills all units within one square of the strike, causes pollution, and reduces the population of any city within the radius by half.
3.12 Barbarians do not always attack and defend at normal unit strengths. Barbarian archers defend with a base value of 1, not the normal archer defensive value of 2. Other barbarian units have normal defense values. All barbarian attack factors are affected by the difficulty level of game being played:
Chieftan - 25% of normal
Warlord - 50%
Prince - 75%
King - 100%
Emperor - 125%
Deity - 150%
3.13 Sneak attack! A sneak attack gives an advantage to the attacker. The value of this bonus is currently unresolved (see the thread Does it pay to be sneaky?).
First, some background. The manual provides a simplified combat formula to approximate the likely outcome of a combat. This formula states that the total modified attack and defense factors are combined; The probability of victory is approximately the ratio of the unit's factor to the total. In other words,
U / (a + d)
where
U = the unit's modified factor,
a = the attacker's modified attack factor, and
d = the defender's modified defense factor.
This approximation has proven insufficient for many Apolytoners. Much debate has resulted in a more detailed formula. First, an explanation of what it entails. For explanatory purposes, a combat is a single round, a battle the cumulative result of all combats.
The attacker and defender each own a portion of the total modified attack and defense values proportional to their value, as the simplified formula, above, expresses. For each combat, a winner is determined by this ratio. In simple terms, the higher modified value is more likely to win a combat. For each combat won, the opponent reduces its hit points by the winner's firepower. This continues until one unit is reduced to zero hit points and is thus destroyed.
Heated discussion of a simplified combat formula led to several ideas being put forth. The one that most closely gives the results of the complex formula is:
Odds = (S + (S - W)) * Shp * Sfp / (W * Whp * Wfp)
where:
S = Stronger unit's modified attack or defense value
W = Weaker unit's modified attack or defense value
Shp = Stronger unit's hit points
Sfp = Stronger unit's firepower
Whp = Weaker unit's hit points
Wfp = Weaker unit's firepower.
No playtesting results to test the accuracy of this formula have surfaced.
On to the real calculation...
Each unit gets a randomly generated number from 0 to its modified value minus one, multiplied by a constant. This constant has been best-guessed (based on play testing) to be 8. The unit with the higher random number wins the combat, ties going to the defender.
If the defense value is equal to or greater than the attack value, the probability (p) of the attacker winning the combat round is
p = (A - 1) / 2D
If the attack value is greater than the defense value,
p = 1 - ((D + 1) / 2A)
Where A = (a * 8) and D = (d * 8). The resulting p is the odds of the attacker winning the combat. To calculate the odds of the attacker winning the battle, p is plugged into the equation
P = SUMn(COMB(n-1,dh-1) * (p^dh) * (1-p)^(n-dh))
Where "n" is summed from dh to dh+ah-1.
ah = Attacker's modified hit points
dh = Defender's modified hit points
p = probability for attacker to win combat round
P = probability for attacker to win battle
Note: Modified hit points: An opponent's total hit points are divided by the unit's firepower. Thus an attacker with 2fp halves the defender's total hit points.
Comments:
This formula accounts for damaged units, as the input is the unit's current hit point total.
In layman's terms, P is the sum of the probabilities of the all possible outcomes of the attacker winning; For example, the odds of the attacker winning ten straight combats, plus that of winning ten out of eleven, plus that of winning ten out of twelve, and so on, added together to produce a grand total. The maximum number of rounds is the attacker's and defender's total hit points minus one. Basically, until the winner is left with a single hit point after destroying the loser. The defender's chance is (1 - winner's probability).
For those unable to decipher the above mathematics,Units with 0 defense can occasionally win because it generates a number between 0 and 0 (0, thus), and the attacker can also end up with a 0. The tie goes to the defender, meaning he wins that combat round. A damaged unit, or one attacking at partial strength (after using part of its movement allowance), can on rare occasions end up on the short end of this possibility.
Artillery (10a, 1d, 20hp, 2fp) vs. Rifleman (5a, 4d, 20hp, 1fp): The formula predicts 82.5% victories, Marko Polo tests resulted in 79%.
Armor (10a, 5d, 30hp, 1fp) vs. Rifleman (5a, 4d, 20hp, 1fp): The formula predicts 45.8% victories, Marko Polo tests resulted in 44%.
Other tests of Cavalry vs. Armor, Fanatics vs. Musketeer, Fanatics vs. Alpine Troops, Warrior vs. Warrior, Catapult vs. Musketeer, and others all matched the formula closely.
5.1 A city's population is not reduced if the city has city walls and a defender loses a combat.
5.2 A sacked city will have its population reduced by one. A size one city will usually be destroyed if sacked. There is unresolved discussion about when size one cities are captured instead of destroyed. See the thread City Razing Problem for more.
5.3 Naval bombardment ignores city walls.
5.4 Units that 'Can make amphibious assaults' (Marines) can attack from the sea (a ship), but do not ignore city walls. This does not count as shore bombardment.
After the battle, the victor is usually damaged. Units can heal as long as they do not move, not even along railroad. They heal at varying rates, depending on their proximity to a friendly city or barracks. Settlers and Engineers, the only units incapable of fortifying, heal while working.
6.1 Land Units:
6.2 Naval/Aerial Units:
6.3 Healing away from a city or fortress (by skipping that unit's turn) occurs immediately, before the next unit is available for an order. Barracks healing occurs at the end of the player's turn.
As a general rule, the unit with the higher modified attack or defense strength will likely win the combat.
Against 1hp defenders:
Defenders in a city have the advantage of maximum defense and healing. Also, only a diplomat or spy can see what units reside in an enemy city.
Stacked defenders are immune to bribery unless adjacent to a city that is bribed.
Much discussion has surrounded figuring out how combat works. In the process, some misinformation has been spread. This section addresses these incorrect ideas.
The following statements about CivII combat are all FALSE. Where applicable, a reference number before the statement directs you to the section where the correct information can be found: