Master of Orion/General battle tactics

Ship size
Generally speaking, ships are better the bigger they are.

Part of this is because it is easier to get whole kills on small ships. Any hit on a small ship is practically guaranteed to kill it at any point in the game. If the ship dies, its ability to attack is obviously gone. It is, then, easier for the enemy to reduce your ability to damage them by quickly killing a lot of your firepower.

Larger ships can usually take many many hits before they die so it is hard to reduce your ability to counter-attack if you are using large and huge ship designs.

Additionally, the huge ship designs are practically built to use automated repair and advanced damage control. They have the ability to use all the best defensive systems at the same time and to reduce the damage they take massively. The automated repair is based on max hit points, so if they can't do 15% of the hit points of this ship in one turn then they will never kill it. If they can only do 20% damage to it, it will take like 17 turns to defeat the ship. That is a lot of time for your firepower to be going to work. You can retreat the ship when it is close to death and inflict many casualties and have none of your own.

It is very hard to pull this off with tiny and medium ship designs.

Tiny ships do get a +2 bonus to maneuverability, medium get a + 1, and huge gets a minus 1 to maneuverability, but this is partially negated by the fact that bigger ships have room for stuff like Inertial Stabilizer which allows them all to be about the same if necessary. You can't turn it off on a tiny ship though whereas you can opt not to include inertial stabilizers/nullifiers on the huge ships if you want.

The bigger ships can fit more weapons though and other systems that are very hard to get on tiny and medium ships.

If you want to have the best chance of losing absolutely zero ships, you want to definitely overweight the huge designs.

If you want to use High Energy Focus and almost all the other special equipment systems as soon as you get them, you are almost guaranteed to have to build large ships or better.

One thing to note, though, is that in the beginning of the game, sometimes tiny ships are all you can make and get somewhere fast enough to have forces there at all. If a world is pretty newly created it is possible that they can make a couple tiny ships if enemy forces are coming and you don't have the ability to get anything else there at all. This can perhaps save you from having enemy ships end the turn over one of your planets, and it can be worth a try. Last resort potential is there.

The kind of ship that will probably be the most useful for most of the game is the large design, so try to get used to using those as your mainstays.

Scouts as blockers
In the beginning of the game, weaponless scout ships can be very useful in combat.

If an enemy ship has no weapons, it will always retreat if it is the only thing in space combat on its side. This happens very often in the early game when all the ships in the game are scout ships and colony ships.

Thus if you can get to a planet first and leave a scout in orbit over it, you can keep enemies from getting info on the planet. They won't know the environment and planet size if they can never end a turn with a ship over the planet (until they get Advanced Space Scanner).

This is useful because it allows you to make their colony ships turn around and go home and buy yourself time to get colony ships to the planet first. They will have to come back with ships that have weapons in order to get your scout to retreat. Sometimes they might send as many as 3 colony ships without an escort and all of them will have to return home. Other times they will send an escort with the first run. Still, it is worth it to try this, especially on really good worlds, if not every world near you. The less information the enemy has, the better.

Usually, if you have ships over a planet, the enemy will try to expand somewhere else rather than fight you for it. It will usually only fight you for it if it scouted it first and then you showed up to keep bouncing its colony ship or if it has run out of places to expand elsewhere. If it has run out of places to expand elsewhere, you are guaranteed to experience fighting over these planets if there is nowhere else for the computer to expand.

Avoiding invasions
Never, if you can possibly avoid it, let an enemy fleet end the turn with a fleet in orbit over one of your colonies.

At even the easiest game settings the computer will try to invade with colonists if it can do this. In the hard and impossible settings, they are guaranteed to try to invade if this happens.

It is very important to avoid this at all costs. It is not worth trying to setup a trap like this, it is better to not even try to do something like that. Doing so is just going to be taking risks that will only very rarely pay off.

Launching invasions
The ground force invasion is probably the absolute best thing you can ever do on the field of battle. For every 50 or so factories that the enemy has, you often get a technology that they have that you don't have (up to 6).

This has the potential to be the biggest impact to the game of any single thing you can do. The potential exists that you can get 6 techs that are all higher level than anything you have been able to research. You could easily jump yourself ahead in research by 50 turns by doing this. Getting 3 or 4 sets of 6 could potentially move you ahead by 200 turns. That is a game breaking advantage.

If they had 2 or 3 techs that you didn't and that allowed them to keep up with you, and then you take them this way, that could signal a game loss for them on the spot. It is hard to lose when you have all of the enemy's best techs AND you have a bunch of stuff they don't have.

At the same time, you don't want them to be doing that to you if at all possible.

Try to do everything possible to take every world with an invasion if you can. It is only when you have an overwhelming advantage that you should try to press it as quickly as possible and just blow planets to pieces as fast as you can to put them out of the game as quickly as you can. Hundreds or thousands of free factories and the potential for hundreds of free easy to clone population units is just too good to pass up any other time.

One of the biggest advantages of doing this is the fact that every time an invasion happens successfully, they lose one planet and you gain one. They might lose 3% of their production and 3% of their research when they lose a planet and you gain the same. That is a 6% swing in total power from one side to the other. It only takes a handful of worlds to create a 50% swing in total power.

Past the early game, it is almost never worth it to even build colony ships for this reason. Many pros never build another colony ship after the first vote has occurred, relying solely on invasions from that point forward to increase their territory.

The bonus to research alone that you get is awesome from doing this. If you just put the world on research after you max its population and factories that might result in you researching every single tech 3% or 5% faster. It only takes 10 or so worlds before you have gotten the Psilon racial ability for free, maybe even less. In some cases much less (depending on how much you already devoted to research spending on your current worlds).

If you don't control at least about half the planets in the galaxy, chances are good that you should be invading rather than destroying planets to get the biggest possible advantage.

Additionally, incremental gains once maintenance costs are met are the best gains. For you that means you gain massively when you take over a new planet. When the opponents lose a planet, what it contributed to planetary budgeting is now gone and the squeeze is felt extra hard. Taking a couple planets from an enemy can bring their research to a grinding halt because the worlds they have left still have the same expenses as before but less worlds to pay those expenses with. Their spying drops dramatically as they start losing worlds because that is one of the fastest things to go when maintenance costs get huge. Also, it is much harder to spy on you when you have tons of free Computer field techs from invading enemies.

If you are playing on higher difficulty levels, you always always always want to start the invasions first on the small worlds the enemies have if at all possible. Small worlds will only have maybe 20 or 30 missile bases on them whereas some large ones might have 40 or even 60. You will probably not have overly many ships if you are trying to get the offense going quickly and 20 missile bases is much much easier to defeat than 50.

Chances are good that the small worlds will still have the 300 or so factories necessary to most likely get the full 6 techs in an invasion. These worlds are also more likely to be mineral rich and to be able to get up and going quickly and to contribute massively to the construction of larger fleets.

Having the first 3 or 4 worlds you take over be rich or ultra rich is a crazy change in fleet production for you and a crazy reduction in fleet production for them.

It is also much easier to win a ground war over a small planet if the enemy outstrips you technologically. Even if they beat you in ground tech by 30 points, you might still be able to take a 50 population world if you have 200 to 300 ground transports in the air. With that level of disadvantage it is very hard to invade 120-200 population worlds, more so because of how fast they can regrow the population they are losing, but on the small worlds the few people they lose from each invasion hurts their ability to recover a lot more.

If the war is going to play out over a dozen turns, you want them to be gaining 3 a turn instead of 10+ a turn.

Conversely, it is almost always worth it to just flat out blow up a poor or ultra poor planet rather than conquering it with ground forces. It takes so long to build these to full capacity even at the end of the game and then to build planetary shields and missile bases on them that your fleet will be stuck for dozens of turns waiting around for the time when they can move away from its defense. This is time that is better spent on other worlds. Just blow it up at the end of the turn and move on. They will still lose significant production and you won't have gained significant production or research anyway if you would have conquered it.