This is a general overview of how wars would be fought in the distant future, and doesn't actually refer to the game of warhammer, it's purely theoretical.

The Nature of War

War takes two main forms the full out invasion of a planet or system or an insurgency in a planet or system. In an invasion the conflict will most likely be brief but intense. With both sides committing large amounts of resources the fighting will be hard, fast and bloody, with one side quickly gaining the upper hand and crushing the other side. The invasion of a planet will differ depending on the attackers goal. If the aggressor wishes to settle the planet they will want to keep the resources and maybe the population as intact as possible. Example would be a Tau invasion or Imperium suppressing a rebellion. The sides would attempt to use conventional armies, and focus on destroying the enemies military power. The opposite to this would be an Extermination, a Tyranid or Chaos invasion. This would see the widespread use of WMD’s and orbital bombardments in attempt to wipe out life on the planet.
In a insurgency the conflict would be low intensity with dispersed forces and probably for longer periods of time than a planetary assault. Examples of this would be small scale rebellions, infestations of Tyranid or Necron or local enemies like Orks. The goal would normally not be to exterminate the planets population, unless the insurgency was a precursor to a larger invasion.

Logistical Support

Conflict would also be determined by a sides logistics, whether an army could be re-supplied on planet or required re-supply from space. On planet would come from either native production, factories and population or a mass initial delivery of resources. Either of these cases would see a more conventional war we the capturing of territory and the denial of resources to the enemy a top priority. The alternative is off world re-supply, meaning primarily space transports shipping in material. In this case there is no longer a need to control territory on the planet in order to deny the enemy resources but the conflict shifts instead to space where fleets blockading or breaking the blockade of a planet would decided the outcome of the war.

This is just a start there may be more to come regarding fleets and actual ground conflicts, depending on how bored I am in class. Feel free to comment or contribute.