Most warfare is settled through space combat. Though ground warfare is necessary to achieve ultimate victory, landing armies is safer once the fleet has eliminated the threat of enemy vessels intercepting and destroying vulnerable troop transports. Winning battles across the stars is about adapting to the enemy's loadout and using effective counters. Each offensive or defensive component has a counter-measure to it, meaning that component choices are likely to evolve and change as empires encounter new enemies and try to build ships that counter those of the opponent.
Combat
Combat in Stellaris takes place in real-time, out of the player's control save for the emergency FTL order. Though the player can direct a fleet toward or away from enemies, there is no control over individual ships (unless they make up a fleet on their own) or which ships they target.
Each fleet is shown on the screen with a strength number. This number represents the power of this fleet, measured by the combined offensive strength of all of the ships in the fleet. This is a rough guide to a fleet's prowess, but cannot be understood as an absolute determinant in how combat will play out. A fleet that has a lot of offensive weapons that are unsuited for the enemy it is facing (Alien creatures with regenerating hulls or skins, enemies with sophisticated and appropriate countermeasures) may still lose to this enemy even though its strength number is considerably higher.
When a fleet engages an enemy ship, enemy station or alien creature in combat, the player gets an alert and the fleet listing in the Outliner has a red battle icon beside it. Select this fleet and you will see a real-time report of the ongoing combat.
At the top of the results sub-screen is a bar measuring the approximate strength of the opposing sides in combat. This bar changes as combat progresses and the odds shift in favor of one or the other side. As the battle progresses, the report sub-screen shows breakdowns of the effectiveness of the weapons systems in use and how much damage the shields, armor and hulls have taken. Analyzing this data helps determine whether current ship builds are effective against this opponent and may indicate ways to improve.
Damage
Damage by space combat is distributed onto three components: Hull Points, Armor, and Shields. Different weapons do different amounts of damage to each.
Hull Points is the most important attribute for any ship. When Hull Points reach 0, the ship will be destroyed. Furthermore, ships will have their fire rate decreased by a percentage equal to half of the percentage of Hull Points lost. Hull Points are determined by ship size and tech bonuses.
Armor provides an anti-kinetic layer of protection, based on the type of Armor component added, and tech bonuses.
Shields provides an anti-energy layer of protection, based on the type of Shield component added, and tech bonuses. Shields require energy from the ship's reactor and always regenerate, unlike Hull Points and Armor.
When being attacked, Shields will absorb damage first. After Shields are depleted, Armor will be damaged. After Armor is depleted, Hull Points will be damaged. Some weapons are better, or worse, than others at depleting Shields and/or Armor, and some weapons deplete Hull Points directly.
When the Hull, and Armor, are damaged the most common way to repair them is by docking the fleet with a Starbase. It is possible to repair ships other than by docking, such as a ship having the 'regenerative hull tissue' component, or a fleet's commanding Admiral having the 'engineer' trait. However, self-repairing is slower than dock-repairing (2% per day or 60% per month). Defense Platforms have a base self-repairing rate of 2% per day. It takes 30 days before a Starbase starts to repair itself after combat and there not being any hostile warships in the system. Disabled Starbases reactivate after 6 ticks of regen (36 days).
Ship protection modifiers are sparse, only two for each defense layer existing:
| Source | Hull bonus |
|---|---|
 |
+10% |
 |
+25% |
| Source | Armor bonus |
|---|---|
 Volatile Reactive Armor edict
|
+25% |
 Matter Compression repeatable technology
|
+5% |
| Source | Shield bonus |
|---|---|
| Exotic Gases for Shield Boost edict
|
+25% |
 Shield Harmonics repeatable technology
|
+5% |
Accuracy, Tracking, Evasion
Three parameters determine the chance of a weapon to hit its target: the accuracy and tracking of the attacker, and the evasion of the defender.
Accuracy comes primarily from the weapon stats. Accuracy is the percentage chance to hit a target with no evasion. Weapons with high accuracy tend to have comparatively low damage or other weaknesses. Accuracy is capped at 100.
Tracking comes primarily from the weapon stats as well. Tracking will cancel out some or all of the defender's evasion, and the actual chance to hit is the attacker's accuracy minus the defender's remaining evasion, if any. Smaller weapon sizes usually have significantly higher tracking, with some exceptions.
Evasion is primarily based on hull type and is a defining feature of the Corvette, Destroyer, Transport and similar small ships. It can be increased by thrusters, by the combat computer for some hull sizes, and by Auxiliary modules like the Enigmatic Encoder and by having surplus power in the ship design. The evasion rate is capped at 90%.
The formula to calculate the chance of a specific attack to hit the target is (remember that evasion is capped at 90 and accuracy is capped at 100):
[math]\displaystyle{ \text{chance to hit} = max(0, \text{accuracy} - max(0, \text{evasion} - \text{tracking})) }[/math]
Generally, the goal is to pick a weapon that has just enough tracking to maintain the weapon's baseline accuracy, i.e. tracking equal to the enemy's evasion. Thrusters and Sensors mostly cancel each other out when on the same level, so without a serious tech disadvantage, the hull size and weapon sizes matter the most. Large weapons sport much higher damage than smaller weapons, allowing them to destroy their intended targets in just a few shots. However, since said large weapons also tend to have low tracking, ships with high evasion may become nigh-invulnerable to them as they can simply dodge the shots. Weapons with higher tracking, such as small weapons or missiles, are needed when engaging ships with high evasion, such as Corvettes. Conversely, small weapons have high tracking, ensuring that they are able to hit their target without missing much at all. However, they also have low damage and therefore struggle to deal with larger ships such like Cruisers and Battleships due to them having very high shield/armor/hull values, allowing them to absorb shots from small weapons without much difficulty. Large weapons with high damage are needed for dealing with ships that have such high durability.
Combat Computers
When in combat, every military ship will advance to within a certain range of its intended target and fire its weapons; combat computers determine what range the ship will engage its target from when in combat. Bold ships will use the combat computer of that behavior by default when the design is auto-generated. Ships that have similar combat computers tend to cluster together, stay in formation with each other and attack in groups.
| Behavior | Ship class | Description |
|---|---|---|
| Swarm | Corvette | The ship will charge straight at enemies and try to deal as much damage as possible from point-blank range. |
| Picket | Corvette, Destroyer, Cruiser | The ship will move to close (30) range and attempt to intercept the enemy. |
| Line | Destroyer, Cruiser, Battleship, Star-Eater | The ship will advance to medium (50) range and hold formation. |
| Artillery | Destroyer, Cruiser, Battleship, Titan, Juggernaut, Star-Eater | The ship will stay at long (80) range and fire its long-range weapons on the target. |
| Carrier | Cruiser, Battleship, Titan, Juggernaut, Star-Eater | The ship will stay at extreme (150) range and fire its long-range weapons on the target. |
| Ignore | Colossus | The ship will ignore any attackers and carry on with its current task. |
Force Disparity
Force Disparity is a combat bonus applied when a smaller force is engaged with a larger one in battle (clarification required: is a smaller force one with fewer ships, lower total command limit points or military power? The English is ambiguous, but fewer ships makes most sense due to the target rich environment, whilst smaller ships evasion would mitigate the rate of fire effect) and gives a bonus to the 
Force Disparity will never make a smaller force win unless it has a significant technological advantage. It will, however, ensure that all but the most overwhelming forces will take casualties and bear a cost for their victory.
Ship disengagement
Any ship that takes hull damage while already below 50% hull points has a chance to disengage from battle, scaled with the amount of damage inflicted. A ship that disengages will instantly leave the battle and rejoin the fleet at the end, until then appearing in the combat interface with a flag icon indicating that it has disengaged. If a fleet contains only disengaged ships it will make an emergency retreat.
Ships will never disengage if the owner uses the No Retreat War Doctrine policy. The presence of a black hole reduces disengage chance by half.
Colossi and Juggernauts cannot disengage.
The chance for a ship to disengage after a hit is determined by:
[math]\displaystyle{ \text{chance} = \frac{\text{damage}}{\text{hull}} \times 1.5 \times \text{friendly} \times \text{ship multiplier} }[/math]
damage: Amount of damage the hit inflicted
hull: The ship's maximum hull points
friendly: 1.25 while in friendly space, 1 otherwise
ship multiplier: Ship type multiplier
| Ship multiplier | Ship type |
|---|---|
| 2 | Science Ship |
| 1.5 | Destroyer, Cruiser, Transport Ship |
| 1.25 | Battleship, Titan |
| 1 | Corvette, Construction Ship |
| 0.5 | Colony Ship |
| 0.2 | Meteor Colony Ship |
The chance for ships to disengage can also be affected by the following modifiers:
| Source | Modifier |
|---|---|
 Hit and Run War Doctrine policy |
+0.33 |
 Trickster admiral trait |
+0.25 |
 Erudite admiral trait |
+0.1 |
 Communications Jammer starbase building |
-0.2 |
 Subspace Snare titan aura |
-0.2 |
 Hell's Heart admiral trait |
-0.33 |
| Unyielding admiral trait |
-0.33 |
As an example, if a battleship with 1300 of 3000 hull points left took a 300 damage hit while under the influnce of an admiral with the Erudite trait (+10%) and a hostile Subspace Snare titan aura (-20%) while outside of friendly space:
[math]\displaystyle{ 0.16875 = \frac{300}{3000} \times 1.5 \times 1 \times 1.25 \times (1 + 0.1 + -0.2) }[/math]
The battleship would have a 16.875% chance to disengage after the hit
If this battleship were to instead take 300 damage hits until it either disengaged or was destroyed on the 5th hit, it would have a likelihood of 52.255% to disengage before destruction.
[math]\displaystyle{ 0.52255 = 1 - (1 - 0.16875)^4 }[/math]
Emergency retreat
All ships of a fleet in combat have the opportunity to trigger an emergency retreat in the middle of a battle. This is a sudden escape using each individual ship's equipped FTL engine whether it is a Hyperdrive or Jump Drive that allows the fleet to escape destruction. The order is given by pressing the Retreat button in the combat sub-screen and the retreat can be triggered even from deep within the system. While on Evasive stance, a fleet will automatically retreat once the option is available. AI fleets in neutral or hostile territory will always retreat if half of the fleet is destroyed.
There are some tradeoffs, however:
- The fleet will go "missing in action" upon a retreat for a number of months, depending on how far the ship was from its home starbase. As such it can be neither controlled nor attacked while moving back to its home starbase.
- Ships require 30 days after entering combat before they can retreat.
- The shock of the emergency retreat causes each ship to have a 10% chance to take 20% hull damage. If a ship would be destroyed by this, it has a 50% chance to survive at 1HP instead.
- There is a 1% chance for each ship to be destroyed when retreating.
The chances to take damage (both the 10% chance to take 20% hull damage and the 1% chance to be destroyed) can be reduced by Emergency FTL Damage Risk modifiers:
| Source | Modifier |
|---|---|
| Hit and Run War Doctrine policy |
-25% |
| Trickster admiral trait |
-15% |
 Caravan Counter admiral trait |
-10% |
The 30 day charge time can be increased by Emergency FTL Jump Cooldown modifiers:
| Source | Modifier |
|---|---|
| FTL Inhibitor starbase aura |
+100% |
| Subspace Snare titan aura |
+100% |
 Black hole |
+50% |
| No Retreat War Doctrine policy |
+50% |
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