Sup folks, invasion in Disharmony can be kinda complicated. I wanted to give my opinion on things and maybe shed some light on the issue.

When you start out you are unable to invade enemy systems because the only ships you have access to have Military Power (MP) which is required for space battle but dont have any Siege Power (SP) which is needed in order to invade a planet. All you can do at first is sit in orbit around an enemy system and blockade it, if you try to invade it with MP fleets alone you ll get a "999" duration until taken over.

You cannot start to invade an enemy system until you have cleared the orbit of all enemy MP fleets.

(blockading is a great option to prevent the AI/enemy from tapping into that system resources)

The tech you need to unlock to get access to siege weaponry is in the upper research tree called "Specialized Isotopes" and will give you access to a siege module you can then install on your ships. You will find the module under your "support" tab. There are a number of techs further "up" that tree which will give you more powerfull siege modules and invasion options.

Now you have the choice to install MP and SP modules but if you do that you ll end up with a ship design which isnt good at either area. If you go for full SP ships then yes....you ll end up with fleets which are great at attacking systems but cannot defend themselves. Thats why pure MP fleets are still required to clear the orbit and keep your invasion fleets save (enemies cannot directly attack your invasion fleets as long as you have a MP fleet in orbit, only when those are destroyed are invasion fleets valuable targets)

So the basic procedure is....

- move a SP fleet into orbit of the enemy system
- clear the enemy orbit of any MP fleets
- start invading

The game will now compare your invasion power to the systems defense power. This number depends heavily on the system population and the built improvements there. The difference will return you a number of turns you need to "sit" there until the system shifts to your empire. During those required turns you need to keep defending your SP fleets and block and enemy attacks trying to "free" the system. The system can still produce stuff but the AI/enemy cannot access the systems resources anymore when blocked. Depending on the system population increasing or the system building more defense buildings during the siege that number can even increase.

You can move more then 1 SP fleet into orbit and the SP will be added to the calculation so a single fleet might result in 5% take-over per turn and another fleet can up that to 10% (there is a cap on % per turn take-over depending on game speed)

Advanced tip: if the enemy system is the only system producing a certain strategic resource and you block it the enemy now cannot build modules which require that resource until freed. So if you plan your move carefully you can cripple an enemy in a single blow. On the other hand you can see how important guarding certain systems of yours can be. The Enemy likes to go after specific systems which will hurt you the most.

Once the system shifts you will have a new system under your command with all the population and improvements it had at the moment of capture. Yes...that means you can have improvements which you normally cannot build or research yourself (special race buildings for example). That system will have 0% control at first and therefore will be a drag on your empire not producing anything and breaking your morale down until you have it stabilized (at around 75%+). You gain control automatically for every turn the system is under your control and you can up the speed of % gains by building certain buildings (check the tooltips).

Its an arms race between a fleets siege power and a systems defense power. You can unlock specialized invasion tools to soften system defenses (Bomb module + Bombers, you need BOTH modules in one ship) or instant capture enemy systems (Troops). Once you start invading a system with any of the mentioned modules on your fleet you will get additional options to chose from. If you can use those options or have success with them again depends on the system defense. If its too high you cannot use a bombing run or your troop invasion will fail. A system can build special buildings defending specifically versus troops or bombing.

If you get a message that your siege power isnt high enough to use a certain tool continue piling more SP fleets on that system until you can. ALL fleets in orbit are part of an invasion strike meaning if you have 5 SP fleets and 1 MP fleet in orbit and only ONE of em has a bomb module installed the total of the fleets SP power will decide if you can use that option or not. Also if you manage to bomb the system all actions of all the orbiting fleets are spent on that turn (even your MP fleet cannot move or attack).

EDIT: MP fleets now keep their movement and action points after a successfull invasion via troops and can be used as usual.

EDIT: further testing by me shows that in endgame after your opponents unlocked and built certain defense tech their systems become invulnerable to Bombing. I piled in excess of 6 million siege power on a system with a defense power of ~44.000 and my option was still greyed out.

Its kinda realistic too in a sense. Once technology evolves so much and you channel most of your efforts into protecting your empire it becomes immun to blitz strikes. As you can see I can still flip it the old-fashioned way (which gives my opponent time to react tho) or try to land troops (which is useless, I tried 4 times always increasing my siege power, ended up with 10 million siege power and lost all my troops every time)

EDIT: I realized by now that there are 3 different defensive values playing into defense which require concentrated specialized invasion designs to overcome. Usually my invasion fleets consist of a LOT of invasion modules and then maybe 1 or 2 troops / bombers to add options. That means that my invasion fleet is STRONG versus pure system defense and WEAK against AA emplacement. If I only bring bombers the situation will change and my chances increase.

If you are using troop modules you will most likely lose troops in an invasion being subtracted from your fleets reserve. Once you go down or reach zero your invasion capabilities will reduce or will be off completely until restocked. You can "refill" troops in 3 different ways (the same counts for fighters and bombers)

1. build new ships and move em to that fleet replacing spent ships
2. change the ship design slightly then "retrofit" that fleet bringing all its modules to max (you can only do that in orbit of a friendly system with a min% of control and you need to have access to the resource required....if the enemy blockades that system you cannot retrofit ships you have on it)
2b. by retrofitting and refilling your troop ships the population required will come from the system you orbit at the moment. If the system doesnt have enough population to completely retrofit ALL your ships in your invasion fleet it ll fill up whatever it can (going to 1 pop per planet in the process and leaves the rest untouched)
3. make sure you have a repair module installed. This will slowly "regenerate" spent troops and fighters/bombers automatically over time.

Thats all I got folks :)

EDIT: mentioned before I wanted to go into detail about the 3 different defense stats applied in the game. All system defense improvements have to be unlocked via research.

1. systemd efense
is your "basic" stat that is increasing in big chunks and mostly depends on your population. This counters regular invasion modules.

2. Shelter
is an improvement that "hardens" your population against troop invasions reducing collateral damage. This counters troop invasions.

3. AA emplacements
prevent enemy aircraft from entering the atmosphere to drop their payload. This counters Orbit bombardments.