3.1.3 Combat

Combat occurs when a unit of one civilization attempts to enter the square of a unit of another civilization. If not prohibited by various restrictions (e.g., a land unit may not attack an air unit), combat takes place. In the course of combat, either one unit or the other is destroyed, or both, and the survivor may receive damage as well. The outcome depends on several factors, including chance.

Each unit type has several numerical characteristics relevant to combat: Attack strength, defense strength, hit points, and firepower. See Unit Types for these values. A particular unit may have additional attributes that bear upon its ability in combat: Veteran status, current hit points, and movement points remaining.

A unit with zero attack strength or zero movement points may not attack. A unit with zero defense strength is simply destroyed when attacked, with no loss to the attacker (except the expenditure of the movement point). Air units outside of cities can only be attacked by Fighters and Stealth Fighters. Land units may not attack sea units on the Ocean. When a unit attacks a square, it does combat with the unit that the game considers best able to defend; all combats are one-to-one.

To resolve a combat, first the attacker's adjusted attack strength is determined:

• If the attacker is a Veteran, its attack strength is multiplied by 1.5.
• If the attacker has only one-third of a movement point remaining, its attack strength is divided by 3; if it has only two-thirds of a movement point, its attack strength is multiplied by 2/3. (The client will warn the attacking player that either of these is the case, and invite the player to cancel the attack.)
• If the attacker is damaged, its attack strength is multiplied by (current hitpoints divided by maximum hitpoints).

Then the defender's adjusted defense strength is determined (all of these are cumulative):

• If the defender is a Veteran, its defense strength is multiplied by 1.5.
• The defense strength is multiplied by the defense multiplier of the defender's terrain; see Terrain.
• If the defender is Pikemen defending against a horse-based unit (Horsemen, Chariot, Crusaders, Knights, Dragoons, but not Cavalry), or an Aegis Cruiser defending against air units (including missiles and helicopters), the defense strength is doubled.
• If the defender is in a city with a SAM Battery and the attacker is an air unit (not a helicopter) or a missile, the defense strength is doubled. If there is an SDI Defense in the city and the attacker is a missile, the defense strength is doubled again.
• If the defender is in a city with a Coastal Defense, and the attacker is a sea unit, the defense strength is doubled.
• If the defender is in a city with City Walls, and the attacker is a land unit (other than a Howitzer) or a helicopter, the defense strength is tripled.
• If the defender is in a Fortress (and not in a city), its defense strength is doubled.
• If the defender is a land unit, and is either in the Fortify state or inside a city, the defense strength is multiplied by 1.5.

If, after these adjustments, the attacker has an attack strength of zero, it automatically loses, and its hit points reduced to zero. If the defender has an adjusted defense strength of zero, it automatically loses, its hit points reduced to zero.

After these calculations, combat occurs in rounds that continue as long as both units are still alive (i.e., have at least one hit point). In each round, a random number is generated between one and the sum of the attacker's and defender's adjusted strengths. If this number is greater than the defender's adjusted defense strength, the defender loses hit points equal to the attacker's firepower; otherwise, the attacker loses hit points equal to the defender's firepower (unless the defender is a sea unit inside a city, in which case the attacker loses only one hit point, because ships in port are unable to maneuver effectively). The first unit to reach zero hit points loses the combat.

Whichever unit survives the combat has a 50% chance of achieving Veteran status if it was not Veteran before, or a 100% chance if the unit's owner possess the Sun Tzu's War Academy Wonder.

If the attacker wins, and the defender is not inside either a city or a Fortress, all other units in the defender's square are destroyed. If the attacker is a land unit and wins, and the defender is in a city lacking City Walls, the city's population is reduced by one.

The act of attacking uses one movement point (or any remaining fraction), but some units (Bomber, Stealth Bomber) expend all remaining movement points by the act of attacking. Cruise Missile and Nuclear units are destroyed in the act of attacking. Otherwise, units with movement allowance remaining after combat are free to move or engage in combat again, as their movement points permit.

Units do not attack enemy cities, but the attempt to move into them attacks instead any enemy units within the city; if there are no units present, moving into such an undefended city captures the city (but sea and air units may not do so).

Loss of hit points due to combat reduces the unit's ability to move (see Movement. It also leaves the unit more at risk in future combats, since it will begin the combat with fewer hit points remaining. Hit points are restored as described in Movement.

3.1.4 Nuclear Combat

Attack by a Nuclear missile is completely different from ordinary combat. A Nuclear missile attacks by moving into the square of the enemy unit or city. If the city contains an SDI Defense, or if the target unit is within the usual area of influence of a city with an SDI Defense, the Nuclear is destroyed and the defender is unhurt. Otherwise, a Nuclear blast occurs, and does the following to each square in a 3 by 3 square area:

• All units in the square are destroyed, even friendly ones.
• Any city in the square has its population halved.
• If there was no city in the square, and it is not Ocean or Tundra, there is a 50% chance that it becomes polluted.