User Tools

Site Tools


If you're new to USC, you might want to check out the Step-by-Step Tutorial, too!



Combat is obviously the most important part of the game. Understanding the combat mechanics in detail is one of the key steps towards developing a good battle strategy. It is highly advised to carefully read this chapter even if you're not really into numbers. On the other hand, none of this information is required to be able to equip a squad and start experimenting with how things work – in real action!

Combat properties

The USC combat system uses many different variables to evaluate the result of an attack. As there's always an attacker and a target (defender), combat variables can be divided into attack and defense properties, the latter being reasonably fewer.

Defense properties are (properties of target/defender):

Attack properties form two major groups, one being the attributes of the attacking unit (attacker properties), the other the properties of the weapon / ammo / attack mode used (weapon properties).

Attacker properties are:

Weapon specific attack properties are:

Attack mode /+ ammo specific attack properties are:

Although not a property, but worth listing:

The Attack Value

The AV is a complex variable that represents the ability of the attacker to hit the target. Doing so in melee combat or from a distance is totally different, thus the calculation of the AV is not the same. AV is then subtracted from the target's Defense Value to get the To-Hit Value, against which the result of a 20-sided die roll (1-20) is tested to determine whether the attack was successful or not. This means that even under the exact same circumstances attacks have a chance to be “more or less precise”, to hit or to miss.

Melee Precision (Base Attack Value) = Melee combat %/20 (0-5) + Reaction %/20 (0-5) + attack mode (weapon) precision (0-5).

Ranged Precision has two components. Static Precision measures the basic accuracy with the weapon, based on proficiency and “static” circumstances, such as target range, light level, or obstacles in the way. Dynamic Precision refers to the attacker's ability to aim at moving or otherwise “erratic” targets, where very quick adaptation is required. The values are independent of each other per se; i.e. having good reflexes doesn't help to hit a faraway target, while being able to aim precisely at a static target doesn't necessarily mean an ability to hit fast, moving ones. (Note: the latter ability is less important, generally speaking, as the top speed of units is limited.)

Ranged Static Precision = corresponding Small/Heavy arms %/20 (0-5) + attack mode (weapon) precision (0-5) {+ bonus from targeting computer (4/8)*}.

* If enabled, gives +4 SP under normal circumstances and +8 SP in darkness or against a cloaked unit (~ counters target's Static Defense Value only from these sources).

Ranged Dynamic Precision = Reaction %/20 (0-5).

The Defense Value

It measures the target's ability to defend itself against the attack by means of dodging, evading, or deflecting. DV does NOT protect against damage but gives a chance to avoid being hit. Defense is successful if the To-Hit Value (DV decreased by AV) is the same as or higher than the To-Hit (Attack) Roll and the attack was NOT a critical hit (see later).

Melee Defense Value = Melee combat %/10 (0-10) + Reaction %/20 (0-5) + current speed (0-5) + cover (0-6) {+ Defense Mode (0-27)} {+ cloaking (8)}.

In ranged combat, the attacker's Static Precision is subtracted from the SDV, and the Dynamic Precision is subtracted from the DDV (both are clamped at 0), then these two results are added together to get the To-Hit Value.

Static Defense Value (SDV) = range (1/grid) + cover (0-10) + darkness (0-10) {+ cloaking (8)}.

Dynamic Defense Value (DDV) = Reaction %/20 (0-5) + current speed (0-5, capped at range).

Critical hits

As stated above, by standard an attack is considered a successful hit if the result of the Attack Roll (1-20) is greater than the To-Hit Value, which is the target's Defense Value decreased by the attacker's Attack Value (Precision).

There is a very important exception, though. Every attack has a chance to score a “critical hit”. Critical hits are by definition considered successful hits, absolutely irrespective of the To-Hit Value. This chance is determined by the Critical Threshold, a value that is based on the actual weapon's own Critical Threshold attribute and a few modifiers (proficiency, active mods, etc). The value of the Critical Threshold is generally between 13 and 20. If the Attack Roll is equal to or greater than this, the attack is a critical hit. Critical Chance is the exact same thing, just expressed in the other way: a CT of 20 means a CC of 5% (1:20), 16 means 25% (5:20), and so on.

Beside being a granted hit, critical hits have other bonuses. They multiply the Damage Factor (default 1.0×) of the hit by 2.0 (martial & small arms) or 1.5-4.0 (heavy arms), which results in increased damage. (Prototype weapons may have a further bonus.) In addition, a critical hit always damages the target's armor by 2.

Critical hits cannot be fully averted, but Melee combat mastery decreases the attacker's chance in close combat by 5%, and the usage of a cloaking device reduces their Total Damage Factor to 0.35× (0.6× against heavy arms). Also, on lower difficulty levels lesser aliens have no chance of scoring a critical hit.

Critical hits have a chance to return some APs to the attacker. This effect is called critical AP-rebound. It symbolizes situations where the attack could be conducted faster than it was expected or where the result of the attack altered the combat situation in a sudden, but positive way. The chance for AP-rebound is Reaction % per hit (area attacks thus have good AP-rebound effectiveness). Only hitting units can confer bonus APs. The amount of APs returned per unit can be 1-4, based on luck and the Reaction skill.

Doing damage

Each attack mode has a maximum damage range, shown as e.g. 10-20. This is the maximum base damage: the maximum damage the weapon can do with the default 1.0× Damage Factor. Based on the respective combat skill, attack modes also have a minimum damage. It is also given in a range, the lower bound being the absolute minimum damage you can do. Thus, in fact, the real damage range of an attack is between the lower bound of minDmg and the upper bound of maxDmg.

After the actual base damage is determined by random methods, Damage Factor is applied to it – a multiplier with a default value of 1.0.


  • Using scattering fire modes (sweep) at a minimal distance (1) increases DF by 0.5 (mainly affects the shotgun and the minigun). This is applied before:
  • Critical hits multiply DF by 2 (martial & small arms) or 1.5-4.0 (heavy arms, randomly) (the result will be 2.0/3.0 or 1.5-6.0, see above).
  • 100% of Small arms increases critical hit DF by a further 1.0.
  • Prototypes with the Critical Damage Factor property further increase critical hit DF (by 0.5 or 1.0).
  • Total DF of a critical hit is reduced to 0.35 (martial & small arms, all aliens) or 0.6 (heavy arms) if the target is cloaked.

Then, the resulting total damage (e.g. 24 for a base damage of 12 and a DF of 2.0) is tested against the target's Armor Rating, which reduces it with its current value. It is also called the “Damage Threshold” because attacks with equal or lower damage are totally absorbed.

The final damage is subtracted from the target's Health Points. If it reaches 0, the target is killed/destroyed.

Some attack modes have area damage. The center (actual) target gets 100% of the possible base damage, the others a smaller amount as the damage decays with distance. Note that every target affected gets a distinct To-Hit and Damage roll: some may dodge the attack, some may not, some receive more damage, some less, some a critical, some not. The amount of distance damage decay varies with weapons, please feel free to experiment with.

The damage of aliens in Single Mission mode is reduced to 0.5×-0.9× of damage by rules on difficulties lower than ULTIMATE, respectively.

Additional effects

Units that survive an attack may suffer additional forms of “damage”.

Based on the Armor penetration property, attacks have a chance to damage the current Armor Rating of the target by 1.

The chance for this is ARP : AR (e.g. an attack with an Armor Penetration of 5 has 25% chance to damage an Armor of 20, whereas an ARP of 20 or more *will* damage it). Critical hits automatically damage the Armor by 2. The damage to the Armor is not taken into count until the next attack.

The chance of Armor Rating loss for marines when the armor is penetrated is reduced to 1:3-2:3 on difficulties lower than ULTIMATE. It does not apply to critical hits.

Hits can also “slow down” and/or knockback their target by 1 grid. These effects are based on the attack's impact force [IF]. Any attack has IF % chance to cause 1 Action Point loss for the target's next turn. Hits also have the same chance to knock the target back if it's Stamina is low enough and it is not protected against this effect. If a knockback occurs and there's no more space behind the target unit, it loses 2 (additional) APs for its next turn, instead. Attacks with an area effect may knock the target back with multiple grids, as the targets become a target again while being “moved” in the target area. They may also suffer additional damage this way!

Tranquilization, if the attack mode has any, is applied in two steps. 33% of the effect is applied automatically when the target is hit. The remaining 67% is only applied if the hit does at least 1 damage (i.e. passes through Armor). The tranquilization done to the target is cumulative for the turn, and comes into effect at the beginning of its next turn.

Incendiary attacks (that of with an ignition value) have a chance to set the target ablaze. This chance cumulates with each hit having an ignition value, but only incendiary attacks can possibly ignite the target (i.e. you can “buff up” a target's chance of burning, but you still need another incendiary hit to set it ablaze, normal ones won't do, even with the ignition chance buffed).

Any biohazard or corrosion done by hits simply increases the target's total currently carried biohazard/corrosion by value.

Critical hits can cause bleeding. The amount of this is damage/10 (floored). Bleeding decreases HPs at the beginning of each turn.

Defense mode

Defense mode is a very important tactical option that means using up all remaining Action Points in the current turn to boost Defense Value until the next turn OR enable the possibility of “opportunity attacks” in the enemy's turn. It also reduces incoming damage.

Activating Defense mode for a unit costs at least 5 Action Points under all circumstances, while any surplus APs mean more defense bonus or more “opportunity” Action Points. Going into Defense mode also requires at least 20 current Morale Points! It can be done with any weapon, though there's the aforementioned super-important difference between melee and ranged Defense mode.

Melee Defense mode is in effect when you have a melee weapon or attack mode selected. (Weapons capable of a melee Defense mode are: Katana, Combat chainsaw, Combat rifle–in melee mode, Energy rifle–in melee mode, Energy blade–not in “Overcharge” mode.) Melee Defense mode is passive<, but it confers current Action Points/2 (2-12) + Melee combat %/10 (0-10) + Reaction %/20 (0-5) Defense Value (total 2-27) until the beginning of the next turn of the current player. This can be really crucial to holding off alien swarms. It does not protect against critical hits, though.

Ranged Defense mode applies to any ranged weapon and attack mode. It does NOT give bonus Defense Value. Based on the unit's Reaction, it enables the possibility of automatic opportunity attacks against targets that move into the chosen weapon range in a FREE line of fire–irrespectively of the weapon targeting mode. These attacks use up the standard amount of APs. A unit has as many Action Points for opportunity attacks as it had when initiating Defense mode. A unit can't fire if these APs have been depleted or ammo runs out (no automatic reloading, even if there would be enough APs). Good usage of ranged Defense mode can be also very important, as this way you don't need to give up your defensive positions or walk into a trap, and yet you can still engage the enemy if they attack. It is not as controlled as manual attacking, though, so don't rely on it indefinitely. Also, watch out for possible friendly fire with area damage attack modes. To engage an enemy in Defense mode, a unit must see the target: it must look in that direction (a 90 degrees rule applies).

Both melee and ranged Defense mode reduces incoming damage by a value between Defense mode Action Points/3 and Defense mode Action Points (randomly each attack). This is applied before Armor damage reduction.

BE CAREFUL! You cannot undo the initiation of Defense mode. Once a unit goes into it, it can't act more for the rest of the turn (though it can TURN and switch weapon/attack mode).


Structural map elements (walls, doors, crates, barrels, databanks, crystals) and units can be used as cover against attacks from specific directions, namely, the four diagonal ones. If you stand next to an object or unit, it will mean you cover from the two diagonal directions adjacent to it (if any). If you stand next to two units, it may mean double cover from a direction. Cover rating is displayed on the tactical view as small green shield icons. A single cover means 3 melee and 5 ranged Defense Value. A double cover means 6 and 10 Defense Value. Relying purely on covers is not advised, but in the long run, it can mean a difference if you dodge some attacks thanks to the cover defense bonus.

Thus the chapter on core combat mechanics is concluded. You find additional important information about attacking & combat here: Weapons.

Back to top

← Previous: Inventory

→ Next: Items

Back to Main page

combat.txt · Last modified: 2022/02/04 12:39 by admin