Every planet is presumed to have a planetary militia of some form, be it reservist formations or literal town militias in the Colonies, also things like paramilitary gendarme forces. These forces are nominally not available for war, but rather mobilize and deploy when a planet is faced with invasion.Proposed revision:
Much the same way, a planet or solar system's defense forces also control planet-based craft and weapons, including anti-starship artillery, minefields, theater shields, space stations, and orbital defensive weapon platforms. The quality and quantity of these forces varies by kind of Sector: Home Sectors have the best defenses and Colony Sectors have the least.
A Sector's overall defensive point power is determined by its GDP divided by 2. Therefore a Colony Sector's innate defenses are equivalent to $1,000, a Midrange Sector's is $3,000, a Core Sector $5,000, and a Home Sector $7,000. Each major planet in the sector enjoys a fifth of this as a defensive combat value: a Colony Sector planet has a value of $200, a Midrange Sector's system defense is $600, Core Sector system-planet gets $1,000, and a Home Sector planet gets the maximum value of $1,400.
Note that while an invading force that has seized control of orbital space can give its army superior planet-wide tactical maneuverability, to completely conquer a planet without permitting pockets of resistance to continue fighting will require you to achieve a ratio of at least 3:1 in your favor Thus a colony sector planet will require an invasion force of at least $600 to take over - a Home Sector world will require a full assault of troops with a combined value of $4,200 to conquer.
Moderators do reserve the right to determine exceptions in either direction based upon particular circumstances and situations.
Every planet is presumed to have a planetary defense of some form. There may be reservist formations or literal town militias in the colony sectors. On any world, there may also be paramilitary gendarme forces, 'coast guard' aerospace forces whose normal responsibility is patrolling planetary orbital space, and the like.
In much the same way, a planet may also have emplaced defense batteries or fortifications that can control both ground and space around themselves, theater shields to cover important parts of a planetary surface from orbital fire, space-based orbital weapon stations, minefields deployed to deny certain volumes to a spacefaring attacker, defense installations on conveniently placed asteroids or moons, or any combination of the above.
There may also be other kinds of defenses not envisioned in the above paragraphs, depending on your imagination.
These forces are not available for offensive war, and have no strategic mobility- they cannot be moved from one system to another. They mobilize and deploy when a planet is faced with invasion. They act as a permanent point value attached to the planet or system, which will be fielded against an attacker but cannot be mobilized to attack someone else. The quality and quantity of these forces varies by the type of sector. Home Sectors have the best defenses and Colony Sectors have the least. A nation's richest sectors will have the highest priority for quality defenses, and also be best able to pay for some of the cost of maintaining those defenses.
A sector's overall defensive point power is determined by its GDP. This is then divided in half, between ground and space
. Therefore a Colony Sector's total ground defenses are equivalent to $1,000 and its space defenses are equivalent to $1,000. A Midrange Sector's ground and space defense forces are worth $3,000 each, a Core Sector's are worth $5,000 each, and a Home Sector's are worth $7,000 each. [Note: Fin proposes that different empires might split this differently- I assume he means things like "two thirds of defensive points on ground, one third in space." What do people think of this?]
Each major planet in the sector enjoys a fifth of this as a defensive combat value: a Colony Sector planet has a ground and space defensive value of $200 each, a Midrange Sector's system defense is $600 each, Core Sector system-planet gets $1,000 each, and a Home Sector planet gets the maximum value of $1,400 each.Space Defenses
The space defenses act as a deterrent to casual raiding forces, or to unopposed troop landings. Being sure of beating the defenses in a naval battle requires a task force strong relative to the point value of the defenses. Raiding the system with forces weaker than the defense point value is possible, and you can do damage this way, but there is a risk of losing the raiders, and the defenses will usually stop the raiders from wrecking anything vital.
In general, it's impossible to land a ground force of significant point value on a planet until the planetary/system defenses have been neutralized. However, even once the space defenses are beaten, the planetary ground defenses remain.Ground Defenses
To overcome a planet's ground defenses, the attacker must assault the planet itself by landing troops.
If the point value of the troops being landed is less than the point value of the ground defenses, then the attacking troops will probably be repelled and the invasion force wiped out unless the attacker uses massive, indiscriminate orbital bombardment. Such bombardment has its own repercussions. It invites retaliation, hostile international attention, and moderator sanction.
If the point value of the troops exceeds the point value of the defenses by three to one or better, the invaders can subdue the whole planet in a lightning campaign. This usually means lower casualties in the long run, and is much faster, but also expensive in terms of troop commitment. Any ability of the planetary defenses to cause harm to spacecraft in orbit will be neutralized quickly. A handful of guerillas in remote areas or extremely well dug-in fortifications might last for some time, but practically the whole planet will be in the invaders' hands in short order.
If the ratio of invader point value to defense point value is between 1:1 and 3:1, the invader will need to fight a more protracted ground war. As long as they have orbital superiority, victory is likely in the long run. But there will be large holdout areas with meaningful ground-to-space defenses for some time: the invader hasn't brought enough force to hit every place on the planet at once and subdue it before resistance to the landings can be organized.
Long term occupation of the conquered planet will depend on the size of the population, the size of the occupying army, the locals' political attitudes toward the invaders, the nature of the planetary environment, the strategies used by the occupiers, and other factors. If the occupation force is extremely weak or badly handled, the conqueror may have serious problems with guerillas and resistance movements.
In general, occupation requires much lower point values than invading and conquering the planet.