Soldiering is the strength of soldiers and other resources associated with traditional warfare (ground and aerial vehicles, conventional weapons, tactics, etc.) in a particular race's army. Soldiering allows planetary invasion to be conducted in a more efficient manner, resulting in less friendly losses for more enemy kills. It appears to work by modifying the Advantage Factor of the race.
Soldiering can be improved via race bonuses and certain research technologies. With research in Planetary Invasion tech branch a player also gets access to several invasion tactics, that cost money and may destroy improvements or decrease the planet quality, but improve chances of successful invasion. However, evil-aligned races can construct the No Mercy Invasion Center, which negates the cost of all invasion tactics.
There is a Planetary Defence sub-branch which allows the building of Planetary Defence improvements. There's also 30 points of Soldiering bonus spread across three techs, which can be used offensively or defensively.
After researching Stellar Marines it is possible to construct a Tir-Quan Training center, a Galactic Achievement which improves Soldiering by 25.
Note some AI races (Yor and Drengin) have good starting soldiering bonuses, but they still can not invade planets without the Troops Module, which is made available by researching the first technology in the Planetary Invasion branch.
It is important to note that although soldiering plays a key role when it comes to the planetary invasion, during a planetary defense, soldiering only plays around 33% of the overall mechanic. the other 66% includes planetary defense improvements and the starships currently orbiting the planet. During the invasion, soldering plays around 75% of total mechanic as the higher number of soldiering that you have means the more favorable the outcome that your invasion force will attain. this of course, can be further compounded with invasion tactics. Note: It is never a good idea to trade your race specific soldiering tech with other race as it would further booster their soldiering in addition to the soldiering tech they can get from their own technology tree.
Race specific soldiering technologies in ToA. Terror Drones I, II, III.(Yor Collective) Ultimate Shock Trooper.(Drengin) Inner Magic. (after researching xeno ethnic as Alterian Resistance) War College. (Terran Alliance)