Planetary assaults are complicated if the target is a habitable garden world; the attackers cannot approach the defenders straight on.
The Citadel Conventions prohibit the use of large kinetic impactors against habitable worlds. In a straight-on attack, any misses plough into the planet behind the defending fleet. If the defenders position themselves between the attackers and the planet, they can fire at will while the attacker risks hitting the planet.
Successful assaults on garden worlds hinge upon up-to-date intelligence. Attackers need to determine where the enemy's defenses are, so they may approach from an angle that allows them to fire with no collateral damage. Note this is not necessary for hostile worlds.
Once control of orbit has been lost, defensive garrisons disperse into the wilderness. An enemy with orbital superiority can bombard surface forces with impunity. The best option for defenders is to hide and collect reconnaissance in anticipation of relief forces.
Given the size of a planet, it is impractical to garrison entire conquered worlds. Fortunately, colonization efforts tend to focus on building up a dozen or fewer areas. Ground forces occupy the spaceports, industrial facilities, and major population centers. The wilderness is patrolled by unmanned aerial vehicles1 and satellite reconnaissance. If a defender unit is spotted, airmobile rapid deployment units and satellite artillery are used to pin down and destroy them.