Planetary invasion

Taking planets from enemies while preserving (most of) their civilian population has many benefits: those planets start making credits faster, build large ships faster and contribute more research units than low-pop colonies. But organizing and invasion on a fully-populated homeworld is a rather daunting task that involves 100-200 infantry or 50-100 tanks. Producing them requires lots in-game money and lots of micro-management. This article should make reader understand needed tasks better.

A good article discussing Troops - Max strength, clones vs robotic troops, cost balance, training.. can also be found on matrix forums: http://www.matrixgames.com/forums/tm.asp?m=3999794&mpage=1&#4000506

To-do list for planetary invasion
To successfully invade a planet a player needs to bring enough troops, tanks and/or special forces to enemy's colony. So player needs to:
 * 1) produce invasion units,
 * 2) transport them to enemy's planet,
 * 3) win the ensuing planetary battle and
 * 4) win the planet again if conquered population rebels.

Producing invasion units
Galactopedia: "Troops" and "Ground Combat" offer good explanation on ground units interaction. Reader can also check this site for more data on Troops. What it does not explain is a player should not go to war if he doesn't have either big cash reserve (significantly over 1M with balanced income and expenses) or large positive cash flow (200k+), because just maintenance of 50 armor units will take away 100k, and fleet to fight AI another 150k (numbers from Hard difficulty).

All units have some costs to produce and additional cost to maintain. It could be economically efficient to build units on more than one planet to avoid those maintenance costs of already-built units, that just sit on a planet for several game-years waiting for other queued units to be built. It would also be beneficial to research unit-improvement techs BEFORE units are built, since existing units don't improve with better tech. One additional option to consider is building troops on a planet(s) with a majority of population from a different race. Since races' units have quite different attack strength (Troops - Infantry attack strength), this could save quite a lot of "troops training" (see below).

What invasion force is needed? Player can obtain info on defensive strength of a planet by hoovering mouse pointer over Troops line in the bottom left pane of a selected planet. For a successful invasion about twice as strong force is needed. However if invasion is prolonged, defender will build additional units that strengthen the defenses, so player can consider bringing three (or more) times as strong invading force. Another benefit of bringing significantly more invasion units are additional combat bonuses like "Armored breakthrough" 25% attack bonus.

Training units
One way to make invasion units stronger is to "train" them by invading less-defended planets first, re-invading rebelled planets, or destroy pirates' buildings on player's planets. Since units get experience per battle, will each fight make them some 5-10% stronger, depending on accompanying general's "Troop experience gain" perk. Player should make sure to drop ALL units at once, so even if battle lasts only one day, they are involved and will get the experience.

Lastly, troop generals also affect battles greatly, so they should always be transported (assigned to troop fleet) and dropped with invasion units. BTW generals should always have at least one unit to command, else they'll quickly start developing bad perks like "Drunk".

To summarize: when starting war have lots of cash, build tanks on 2-3 planets, use 3-fold as strong force, and "train" them on easier targets before invading a HW.

Transporting invasion units
Galactopedia has some good info on that regard, however achieving the goal of dropping all troops in the shortest time possible can not be achieved with troops dispersed among many warships, so a specialized design is preferred. A cost-effective design is to put on it as much troop compartments as possible, disregarding all but token weapons and defenses while retaining just moderate mobility. It should never fight, and should drop troops only when space control over planet/system is achieved.

Best way to use it is to merge all troop carriers into just one fleet, that waits outside but close to soon-to-be-invaded system. When space control is achieved, a player should select that fleet from the left fleet pane and just right-click on the planet. If all troop carriers are of the same design, they'll jump in and drop troops and generals almost simultaneously. If troop transports don't want to attack a planet (don't drop troops), they likely have "Do not invade" order active in ship design window.

Win planetary battle
The speed of wining an invasion of any planet depends mostly on three factors: Planet is successfully invaded, when the ratio
 * attacking strength of invading forces as shown in the invasion fleet window,
 * defending strength of defending forces as shown in Troops line in the bottom left pane of a selected planet and
 * attack and defense modifiers, that are applied when ground battle starts and (some) while it lasts, as shown in the planetary battle window.

attacking strength of invading forces * attacking bonuses

-

defending strength of defending forces * defense modifiers

is larger than 20 : 1. So the planetary battle ends when defender has only 5% of the attacker's strength.

Attacker can increase his odds by Attacker can also decrease defender odds by When invading high-population planets is speed of invasion important, because the longer invasion takes, the more invasion units will be damaged or even destroyed. Invader will be slowly losing his advantage in numbers, while defender will be constantly building new defensive troops. Another problem of prolonged war will become population happiness (-xx% "We're tired of war"), which will be getting worse (down to -40%) the longer the player's empire is at war.
 * bringing more and better troops (+25% "Overwhelming odds",
 * clear and hold orbit of invaded planet (+25% "Space control"),
 * drop significantly more Armor units (+25% "Armored breakthrough")
 * drop Special forces ((+25% "Special Forces")
 * bring in general(s) with attack bonuses.
 * assassination of defending generals,
 * dropping also Special forces, which will first attempt to destroy defensive facilities or
 * planetary bombardment, which can quickly reduce amount of defending troops, but also a) destroys facilities, b) does large environmental damage (which after successful invasion "kills" all population over what planet can support) and c) gives player bad reputation, the longer bombardment takes.

Re-conquer rebelled colonies
When planet is taken, player should rest troops to "repair" eventual battle damage (troops only heal on planets, not on ships), load troops and generals and send fleet in the safe location again. If player doesn't want that planet to rebel, he should leave there about twice the number of Infantry as is the population in billions (Planet has 3B pop, so leave 6 Infantries). Some races (mainly insectoids) will nearly always rebel, when the main invasion force is gone. To prevent this, invader should leave all troops there until planet's population calms down, that is when the sum of positive approval modifiers (green lines in planet's population tab) becomes significantly larger than the sum of negative modifiers (red lines).

If AI invades that planet and takes it back, it will be done with small army, so player has 2 benefits: that army has weakened defenses of a planet where it has been taken from, and player got another planet to "train" his troops before main assault.

If planet rebels and it's a high-pop planet and the odds are heavily in favor of rebelled population, and player can not reinforce planet with additional troops, then player should transfer generals from that planet to the closest ship available. After generals are safe on the ship, player should try to extract as many troops in order to preserve their experience.To extract troops back to troop carrier, a ship with troop capacity should be near planet. Player should find the name of that particular troop, open "Troops" tab of the colony, select that troop and click on Transfer button on the bottom right side of that tab.