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A unit cannot enter a square occupied by an enemy unit, and when directed to do so will attack instead, locking the two units in combat until one is destroyed. An attack usually costs the aggressor one movement point, but results in no actual motion — the surviving unit remains where it was when the combat started. Bombers spend all of their remaining movement points when they attack, which gives fighters a chance to intercept them.
Some restrictions upon warfare are rather obvious — units must have a nonzero attack strength to attack, while defenders with zero defense strength lose immediately. There are also limits upon which units can attack which others. Land units can only attack other land units. Ships can attack not only other ships, but any land units adjacent to them (submarines are an exception and cannot attack land units). Helicopters can attack land and sea units and can be attacked by any kind of unit (land, sea, or air), at any time. Bombers and missiles can attack anything on land or sea, and though their targets will defend themselves from attack, they cannot attack the bomber in return. Only fighters can attack every kind of unit.
Note that aircraft within cities and air bases are on the ground, and thus vulnerable to land attack. Ships in port are similarly vulnerable. Note also the special ability of marines to attack targets from aboard ship; other land units must disembark before engaging enemy units.
There are two other actions related to combat. A unit ordered to sentry remains in place indefinitely and no longer asks for orders each turn. Sentry units can not only be reactivated manually (by selecting them), but activate automatically should an enemy unit come into view. Land units can additionally be ordered to fortify, which means they spend one movement point preparing to be attacked; once fortified they enjoy the same advantage as land units within an unwalled city. A unit whose movement points are exhausted cannot fortify — it must have one movement point left at the end of a turn to begin the next turn fortified.
Each unit begins combat with one or more hit points, which are the amount of damage it can sustain. (See the units page for how many hit points each unit starts with, and for the other combat statistics discussed on this page.) Combat consists of successive rounds of violence between the units, which cannot be interrupted and cease only when one unit is reduced to zero hit points and dies. In each round one unit succeeds in wounding the other; the damage a unit inflicts with each blow is called its firepower.
Which unit inflicts damage on any given round of combat is random. The attacker has a chance proportional to his attack strength, while the defender's chance is proportional to his defense strength. For example, archers (attack stength 3) attacking a phalanx (defense strength 2) will have a 3/5ths chance of inflicting damage each round, with the phalanx having the remaining 2/5ths chance. But there are many factors which affect a unit's strengths, which are all summarized in the table below. Notice that many bonuses are possible for defenders, but few for attackers, aside from veteran status; an attacking unit can mostly expect circumstance to work against it.
The normal adjustments to combat strength are as follows:
|General Combat Strength Modifications|
|Unit is a veteran at 1st level (veteran)||×1.5|
|Unit is a veteran at 2nd level (hardened)||×1.75|
|Unit is a veteran at 3rd level (elite)||×2|
|Defender strength adjustments|
|Terrain offers defense multiplier m||×m|
|Terrain includes a river||×1.5|
|In city with SAM battery against aircraft (except helicopter)||×2|
|In city with SDI defense against missile||×2|
|In city with coastal defense against ship||×2|
|In city with city walls against land unit or helicopter (except howitzer)||×3|
|Land unit in fortress||×2|
|Land unit fortified or in city||×1.5|
These factors are combined, thus a city built on a hill with a river that has city walls is very hard to conquer.
There are also combinations of units and circumstances that result in very specific adjustments to combat:
|Specific Combat Modifications|
|Pikemen attacked by horsemen, chariot, knights, or dragoons||Defense strength ×2|
|AEGIS Cruiser attacked by an aircraft, missile or helicopter||Defense strength ×5|
|Fighter attacks a helicopter||Helicopter firepower reduced to 1|
Helicopter defense strength ×1/2
|Ship attacked while inside city||Attacker firepower ×2|
Ship firepower reduced to 1
|Ship attacks land unit||Both have firepower reduced to 1|
Green units have a 50% percent chance of becoming veterans each time they survive combat (33% for hardened and 20% for elite), which gives them greater strength in all future engagements.
Units remain damaged after losing hit points in combat, and will enter subsequent engagements with this disadvantage. Damaged land and sea units also begin each turn with fewer movement points than normal, in proportion to what fraction of their total hit points remain. To regain hit points they must spend turns neither moving nor attacking. Resting in the open restores one hit point; spending a turn fortified restores two; in a fortress they regain one-quarter of their original hit points; and in a city they regain a full third of their original points. The United Nations wonder restores another two points per turn to all of your units.
|Healing Factor Modifications|
|Resting in the open||+1 hp|
|In a city||+33 %|
|In a fortress||+25 %|
|Land unit in city with Barracks||Completely restored|
|Sea unit in city with Port Facility||Completely restored|
|Air unit in city with Airport||Completely restored|
If you sentry a damaged unit, it will become active again and demand further orders when its hit points are fully restored.
You may enhance the effects of a city upon your units with several different buildings:
Cities and FortsEdit
When several units on the same square are attacked, the unit most capable of defense protects the entire square. If the defenders are within a city or fortress, the loss of that defender leaves the other units intact; but outside of such fortification, loss of the defender results in the loss of every unit in the square. Less surprising is that when a ship carrying other units is involved in combat (as either attacker or defender), only the ship participates in the engagement and its occupants are lost if the ship goes down.
Defeating a unit inside a city without walls kills one citizen if the attacker is a land unit. Once the last defender has fallen you may enter the city and claim it as your own with either a land unit or helicopter; ships and aircraft can assault cities but not take them. Upon the capture of a city from another civilization, each building has a one-fifth chance of being destroyed, and the victor may discover a technology held by the vanquished.
Building a fortress requires the construction advance. To begin construction, move settlers, workers, or engineers to the location at which you desire a fort and give them the build fortress order. The work will require only three settler-turns. A fortress can stand anywhere outside of a city.
There are several buildings which enhance the strength of units which are attacked while inside the city:
Nuclear missiles do not engage in combat like other units — they either strike within range of an SDI Defense and are harmlessly destroyed, or detonate and blast the entire 3×3 area centered on the unit or city they attack. Within the blast area all units are destroyed, cities lose half their population, and each land square has a one-half chance of becoming polluted with fallout.
Just as excessive pollution across the world can trigger global warming, fallout raises the chances of nuclear winter with the opposite effect — rather than coastlines becoming jungles and swamp, terrain begins changing into desert and tundra. Settlers, workers, and engineers must be given the clean fallout command to dispose of nuclear waste, which costs three settler-turns per square.