Ground invasions are the usual method of conquering enemy planets. Transport ships carry soldiers from a friendly colony to an enemy world that has been cleared of space-based defenses. 
overhauled the mechanics of ground invasions.
Ground Invasions in Crusade[ | ]
To initiate a ground invasion, load 
Legions onto one or more 
Transport ships and send them to an enemy planet that lacks any space-borne defenses. A fraction of the attacking Legions will be destroyed immediately, according to the 
Planetary Defense statistic of the colony. Subsequently, a ground war ensues, with the colony using its own Legions to defend itself. The 
Soldiering and 
Resistance fractions boost the effective size of the attacking and defending forces, respectively. The players set the initial positions of their Legions and start the simulation, which involves the attacking Legions destroying improvements while taking losses from the defending Legions, If a colony loses all of its Legions, the invaders are victorious.
Invasion tactics[ | ]
The specific tactic chosen by the invader modifies the outcome and aftermath of the battle.
| Tactic | Cost (bc) | Extra Effect | Invader Casualty Rate | Defender Casualty Rate | Planet Damage | Lingering Effect | Technology |
|---|---|---|---|---|---|---|---|
| Conventional Warfare | 0 | None | Low | High | Moderate | None | |
| Planetary Bombardment | 500 | -50% Resistance |
Low | Very High | Very High | -50%  Approval for 50 turns |
 Planetary Bombardment
|
| Information Warfare | 4000 | -25% Planetary Defense and Resistance |
Very Low | Low | Very Low | None | Information Warfare
|
| Biological Warfare | 500 | -50% Planetary Defense |
Low | Very High | Very Low | None | Biological Warfare
|
| Tidal Disruption | 1500 | -75% Resistance |
Unknown | Unknown | Unknown | Unknown | Tidal Disruption
|
| Core Detonation | 1000 | -50% Planetary Defense and Resistance |
Very Low | Very High | Very High | None | Core Detonation
|
Ground Invasions before Crusade[ | ]
How to Invade[ | ]
- First, you must research the
Planetary Invasion technology. - Second, build Transport ships at a friendly shipyard, and launch them with however much population you choose.
- Third, move your Transports onto an undefended enemy world. You'll then choose your invasion tactics, and see the outcome of your decisions.
- Fourth, cross your fingers that you brought enough guys.
- Fifth, profit. Provided you brought enough guys.
Invasion Mechanics[ | ]
All of the invasion events occur behind the scenes. The basic mechanics however have been discussed by the developers in forum posts. Presumably, this is what is happening when you invade a planet.
Invading: Transports bring “Soldiers”; Soldiering modifies the amount of people you bring. So, a transport with 4 people with a 50% soldiering bonus is the equivalent of 6 Soldiers.
Defending: A planet's defense is determined by 3 things:
- The population of a planet is divided by its class, so a class 10 planet with 20 pop, ends up with 2 defending troops per tile.
- Resistance is the % of that population that will fight, if the planet has a 50% resistance, then each tile will have 1 defender.
- Planetary Defense is the most straightforward: a percentage of the invading troops are killed immediately before landing on the planet surface. If 6 soldiers try to invade a planet with 50% planetary defense, half of them die immediately. 6 Invading Soldiers becomes 3.
The invasion then plays out like this…
The invading troops that made it through the Planetary defense system, land and take out one tile at a time. Depending on the invasion, method the casualty rates will vary, but as each tile is fought over, the invaders and the defenders take casualties. For this example we will say the invader has a casualty rate of 50%, which is pretty high.
Tile 1, 3 Invaders > 1 Defender, Invader wins, but loses 1 soldier Tile 2, 2 Invaders > 1 Defender, Invader wins, but loses 1 soldier Tile 3, 1 Invaders <= 1 Defender, Invader loses, invasion repulsed.
This is a really simple example, but is more or less how it works. Where strategy comes in is in the invasion types, as these can modify Soldiering, Resistance, and Planetary Defense.
A few more notes.
Each Transport pod takes its own tile, and moves on, so with more troops and transports the invaders will attack multiple tiles. Number of invaders * soldiering / pods = number of troops hitting a tile at a time.
Unlike in GC2, invasions do not kill everyone on a planet, depending on the invasion type, the defending casualties can vary from light to genocidal. As each tile falls, a certain amount of defenders will survive. If the invader wins, they will become part of the planet's population after the invasion is over.
Some invasion types are so horrific that they will do damage to the planet or at least destroy improvements. They will also have long term effects on the planet, often leaving it with durational modifiers, like low approval for x turns.
Invasion Tactics[ | ]
If you've researched the appropriate technology, such as Biological Warfare, you can pay credits to enable special tactics during an invasion. Biological Warfare, for example, reduces the enemy's ability to resist your assault while simultaneously reducing the chance for existing improvements to be damaged during the invasion.
| Tactic | Cost (bc) | Extra Effect | Invader Casualty Rate | Defender Casualty Rate | Planet Damage | Lingering Effect |
|---|---|---|---|---|---|---|
| Conventional Warfare | 0 | None | Low | High | Moderate | None |
| Planetary Bombardment | 500 | -50% Resistance | Low | Very High | Very High | -50% Approval for 50 turns |
| Information Warfare | 1000 | -25% Defense and Resistance | Very Low | Low | Very Low | None |
| Biological Warfare | 500 | -50% Defense | Low | Very High | Very Low | None |
| Tidal Disruption | Unknown | Unknown | Unknown | Unknown | Unknown | Unknown |
| Core Detonation | 1000 | -50% Defense and Resistance | Very Low | Very High | Very High | None |
Defensive Tactics[ | ]
The ability to resist a ground invasion depends on planetary defenses, resistance, and soldiering ability.
Resistance determines the percentage of the planet's population that will try to defend the planet from an invasion. The rest of the population will suffer losses from the invading force, but they do not assist in repelling the invaders.