Master of Orion II: Battle at Antares/Planning and winning wars

Now fo the fun stuff - you've earned it, after all that careful management of your economy.

Like most of this guide, the pages contained in "Planning and winning wars" are about Medium and larger galaxies - Small galaxies are very different because blitz races are very common there and very rare in other galaxies.

Need to plan carefully
You need to make sure that your campaigns strengthen your empire rather than weakening it. Traps you need to avoid include:
 * Leaving your existing colonies vulnerable. Conquering an enemy colony and losing one of your established colonies is a very bad deal:
 * If you managed your economy well, your colonies are among the most productive in the galaxy.
 * A newly-conquered population sulks, and will not be fully productive for 10 to 60 turns, depending on the size of its population.
 * Your economic and technological strategy is optimized for your original race and may not work so well for other colonies populated by other races. The worst case is usually that if your race is Tolerant you can't research pollution-control techs; and if a conquered colony has a non-Tolerant population (including Natives) and no pollution-control buildings, and the conquest gave you no pollution-control techs, the conquered colony will be severely slowed down by pollution.
 * Needing such a large fleet to conquer enemy colonies and then defend them that the maintenance cripples your finances (10 BC per 1 CP deficit).
 * Needing to feed large conquered populations, which will cost you money and divert people from research (mainly) or production into farming. This is particularly difficult for Lithovores, as they are only allowed to research 1 farming tech, which is fairly expensive to research and requires a fairly expensive building (Weather Controller).
 * Ruining your finances. You may have enough warships but, if your empire is not Telepathic or if your opponent is Telepthic or has a Telepathic Leader, you will need Troop Transports in order to invade, often about 10 to 15 of them in order to conquer enough colonies quickly to hurt your opponent seriously. That requires an additional 10-15 Command Points, which you may not have at this stage in the game - and the defict will cost you 10 BC per CP.

Scorched Earth
If you are trying to win a technology war or simply out-produce or out-grow the enemy you might not even need to retain enemy colonies. Simply destroying them will usually hurt your enemy a lot while at the same time it is much cheaper for your own economy. There is no need to feed the new population, build transports or buy missile bases and marine barracks for the new planets.

It might seem like an evil thing to kill millions of innocent people but then it's just a game and the computer will do the same to you. Killing colonies will hurt your enemy's production, research, cost a lot of money in destroyed buildings and hurt the range of his fleet. At the same time you usually can destroy one planet per turn without the need to wait for transports and move your fleet back to your home worlds once an enemy fleet is on its way with revenge on its mind.