This article is for the PC version of Stellaris only.
Wars are not decided only in space. In order to capture enemy planets, entire armies will be needed, along with the preparation and battlefield awareness to properly use them. However, planets may only be invaded if the system's starbase has been occupied.
Ground combat takes place between the planet owner's defense armies and the invader's assault armies. The number of armies that can be engaged in combat on either side is five plus one-fifth of the planet size. Armies not engaged in combat will be placed a row behind the fighting armies and replace any killed army.
Most armies have a morale value, and once morale drops low enough the army will be negatively affected with the following modifiers:
|50% <||−25% Army damage|
|0%||−75% Army damage|
Armies gain experience from combat and can rank up. Higher rank armies are more effective in combat.
An attacker can always attempt to retreat. However there is a significant chance that each retreating army is destroyed while attempting to return to space, making retreat a risky endeavour and something to do only in very poorly chosen engagements.
|Devastation||Damage to armies||Kills pops||FTL inhibitors||Tree of Life|
Planet devastation is a measure of how much damage has been caused to the planet's infrastructure. It goes from 0 to 100% and every 1% planetary devastation causes a 1% decrease in housing, amenities, trade value, resources from jobs, upkeep from Jobs, growth speed and immigration pull.
Planet devastation is caused by orbital bombardment and assault armies. Assault armies have a Collateral Damage modifier that determines how fast they cause planetary devastation. Defense armies do not cause any.
Planet Devastation recovers by 0.05% daily if planet is not being invaded or bombarded.
Defense armies are automatically recruited by certain jobs based on the species working that job and can be robotic or organic. They are stationed on planets and used to defend against enemy armies. While they are stronger one-for-one than assault armies and their loss does not cause War Exhaustion, their static nature means that investment is necessary to fend off a concentrated attack. Their combat ability is also improved by certain technologies.
The type of army recruited depends on the pop working the building that spawns them. Defense armies do not incur any upkeep beyond that of the building that creates their Job. Defense armies do not cause Collateral Damage.
|Type||Base Damage||Morale Damage||Health||Morale||Source|
|Defense Army||2.25 - 4.50||2.25 - 4.50||250||250||Organic +1 +2 +2 +3|
|Robotic Defense Army||1.50 - 3.00||1.50 - 3.00||250||Immune||Robot +1 +2 +2 +3|
|Undead Defense Army||2.25 - 4.50||3.93 - 7.87||350||Immune||Organic +1 +2 +2 +3 +3|
Replaces normal organic defence armies if at least one necromancer is present
|Offspring-Led Army||2.62 - 5.25||3.41 - 6.82||400||Immune||Progenitor Hive +2 +3|
|Drone Grid||1.65 - 3.30||1.65 - 3.30||220||Immune||Machine Unit +2 +3|
|Sentinels||6.00 - 12.00||30.00 - 60.00||1400||Immune||Archaeology site event outcome|
|Crystalline Bane||52.50 - 105.00||2625.00 - 5250.00||11000||Immune||Unexpected Mineral Seams|
An empire can recruit assault armies on a planet based on the species living there. The species must not have Exempt Military Service rights in order to recruit an army from that species. An empire cannot recruit more assault armies from a species than the total number of pops of that species living in the empire, with the exception of Clone Armies. Armies not tied to a species, such as the Xenomorph Army are unlimited as well unless otherwise specified.
Unlike Defense Armies, assault armies are based in space and each recruited army will board a transport ship. Transport ships are unarmed but automatically upgraded with the latest defense components. Assault armies can be ordered to land on an owned planet to provide additional defense against invasion.
|Regular||0 - 100||None|
|Experienced||100 - 1000|| +10% Damage|
|Veteran||1000 - 10000|| +20% Damage|
|Elite||≥ 10000|| +40% Damage|
As armies fight battles they will increase in rank, improving their stats based on their experience points. Armies get +2 experience points each time they deal damage and +1 when they take damage. Army starting experience can be increased by +100 by each of the following:
- Military Academy planet building
- Dread Encampment planet building
- Strength of Legions civic
- Private Military Companies civic
- Mercenary Liaison Office Corporate Building
Orbital bombardment occurs when a fleet is in orbit around a hostile planet, and is generally a precursor to invasion. Genocidal empires can even opt to skip the invasion entirely and simply exterminate all life on the surface. Orbital bombardment causes damage to any defensive armies as well as Devastation. Effectiveness is based on the Fleet Size of the fleet in orbit, where maximum effectiveness is reached at 200 Fleet Size.
Orbital Bombardment Damage dealt to enemy planets can be increased by the following:
|Overwhelming Force tradition||+20%|
|Orbital Trash Disperser component||+25%|
|Munitions Plant juggernaut aura||+30%|
|Crisis level 3||+20%|
Orbital Bombardment Damage taken from enemy fleets can be reduced by the following:
|Planetary Shield Generator building||−50%|
|Survival of the Fittest tradition||−25%|
|Never Surrender tradition||−25%|
|Fortress World designation||−10%|
|Fortress Station designation||−10%|