Combat occurs between two political entities that are at war with each other. A civilization may be at war with another civ or with a city-state. Barbarians are always at war with all civilizations and city-states.
There are four major forms of combat: melee, ranged, naval, and air combat. The first two occur throughout most of the game, while air combat doesn't happen (naturally) until somebody discovers flight.
War against another civ may be declared in a couple of different ways, or you may find yourself on the receiving end of an enemy's own declaration.
Diplomatically Declaring WarEdit
You may declare war on a civilization through the Diplomacy panel . You may declare war on a city-state by clicking on the city-state's city and picking "Declare War" from the pop-up.
Attacking Another UnitEdit
You can simply order one of your units to attack another civ's units. If you're not currently at war with the civ you're attacking, a pop-up will appear asking if you want to declare war on that civ (or city-state); if you choose to do so, the attack occurs. If you decline, the attack is aborted.
Entering a Civilization's TerritoryEdit
It is also an act of war to enter a civ's territory (unless doing so with a Missionary or Great Prophet) if you don't have an "open borders" agreement with that civ. A pop-up will appear and ask you to confirm your move. Note that it isn't an act of war to cross a city-state's borders, so no pop-up will appear in that case.
An Enemy Declaration of WarEdit
At any time another civilization or city-state may declare war on you. If so, you'll be informed by an unpleasant pop-up (or notification). You may have an opportunity to try to negotiate your way out of the conflict, or you may have no choice but to fight. See the Diplomacy section for details. Barbarians are always at war with you, so you'll never get a declaration of war from them.
Ending a WarEdit
Wars can end automatically when one side has been destroyed because it has lost its last city. Or the combatants can agree to halt hostilities short of this unpleasant eventuality through diplomatic negotiations. You or your opponent may choose to initiate such discussions. See the section on Diplomacy for details. Barbarians cannot be negotiated with. You'll remain at war with them as long as they're around.
Which Units Can FightEdit
Any military unit may attack an enemy unit. Scout units can initiate combat and defend against enemy attacks, though they are weaker than other units. Non-military units such as workers, settlers, and great people may not initiate attacks. If attacked while on their own, workers and settlers are captured (captured settlers turn into workers) and great people and work boats are destroyed. A city may attack an enemy military unit that is within the city's Ranged Combat Range, and a unit may in turn attack an enemy city.
Unit Combat StatisticsEdit
A military unit's combat abilities are determined by its combat statistics. There are four basic combat stats: hit points, combat strength, ranged combat strength, and range.
Ranged Combat StrengthEdit
Only units able to engage in "Ranged Combat" have this stat. It is the ranged unit's combat strength when it is attacking.
The unit's "range" stat determines the distance at which a unit can launch a ranged attack. A range of "2" means that the target can be in an adjacent tile or one tile distant. A range of "1" would mean that the target had to be adjacent to the attacker.
All military units have this stat. Melee units use their Combat strength when attacking or defending. Ranged units use their Combat strength when defending.
A unit's health is measured in "Hit Points". When fully healthy, all combat units have 100 hit points. As it takes damage, it loses hit points. If a unit's hit points reach 0, it is destroyed.
Units receive a variety of benefits during combat, some from the unit's location, others from its defensive posture, and others from a variety of special circumstances. Some bonuses apply only to an attacking unit, some only to a defending unit, and some might apply to both. The most common bonuses come from the terrain the unit occupies, and whether the defending unit is "fortified."
Defending units get important bonuses for occupying forest, jungle, or hill tiles. Attacking melee units are penalized if they attack an enemy across a river. Attacking units get bonuses when attacking from a hill. See the section on Terrain for more details.
Once a civ has acquired the Engineering technology, workers can construct "forts" in friendly or neutral territory. Forts provide a hefty defensive bonus to units occupying them. Forts cannot be constructed in enemy territory. They can be constructed atop resources.
Many units have the ability to "fortify." This means that the unit "digs in" and creates defensive works in its current location. This gives the unit certain defensive bonuses, making it much tougher to kill. However, fortifications are strictly defensive: if the unit moves or attacks, the fortifications are destroyed. While fortified, a unit will not activate. It will remain inactive until you manually activate it by clicking on the unit.
Which Units Can FortifyEdit
Most melee and ranged units can fortify. Non-military, mounted, naval, armored, and air units cannot fortify. These latter units can "Sleep," which means that they will remain inactive until attacked or you manually activate them, but they do not receive the defensive bonus.
The amount of the bonus depends upon the length of time the unit has been fortified. The unit receives a 25% defensive bonus on the first turn it is fortified, and a 50% bonus during all subsequent turns.
The "Alert" OrderEdit
The "alert" order is similar to "fortify," except that the unit will "wake up" when it sees a nearby enemy unit. The wakened unit retains the fortification bonus as long as it doesn't move or attack (so if you order it to go into alert mode again or to pass its turn it keeps the bonus).
Like land units, there are military and non-military naval units, as well as ranged, and melee naval units. Work boats are destroyed when attacked by barbarians and other civs or city-states, and any "embarked" land units are weak, although they can absorb a hit or two.
Ranged naval combat units may attack other naval units and any land units within range that they can see.
Melee naval combat units may directly attack other naval units, as well as coastal cities. There are however certain late-era naval units that merit special attention: the carrier, missile cruiser and the submarines. For more detail, you can view these units by clicking the "Units" tab in the Civilopedia.
Great Generals are "Great People" skilled in the art of warfare. They provide combat bonuses - offensive and defensive bonuses both - to any friendly land units within two tiles of their location. A Great General itself is a non-combat unit, so it may be stacked with a combat unit for protection. If an enemy unit ever enters the tile containing a Great General, the General is destroyed.
A Great General gives a combat bonus of 15% to units in the General's tile and all friendly land units within 2 tiles of the General.
Great Generals are created when your units have been in battle and also can be acquired when you unlock the "Warrior Code" social policy. See the section on "Great People" for more details.
A fully healthy unit has 100 "hit points" (HPs). When a unit takes damage during combat it loses HPs, and if it reaches 0 HPs, it is destroyed.
A unit that has taken damage is weaker than a healthy unit, and it is closer to destruction. Wherever possible, it's a good idea to "rotate out" damaged units from battle to allow them to heal up before reentering the fray. This, of course, is not always possible.
To heal damage, a unit must remain inactive for a turn. The amount of damage that a unit heals depends upon the unit's location.
In a City: A unit heals 25 HPs per turn.
In Friendly Territory: 20 HP per turn.
In Neutral or Enemy Territory: 10 HP per turn.
Note that certain promotions will accelerate a unit's healing rate.
Naval units cannot heal unless in Friendly territory, where they heal 20 HPs per turn.
The "Fortify Until Healed" ButtonEdit
If a unit is damaged, the "Fortify Until Healed" button appears in its Action buttons. If you click on this button, the unit will fortify and remain in its present location until it is fully healed. See the section on Fortifications for details of the defensive benefits of fortification.