Codex: Chaos Daemons (6th ed.), pg 19

A daemonic invasion is all but impossible to stop by conventional means, for the very act of warring against Daemons feeds their power with fear and hatred. Only the closing of the rift can deprive the Daemons of their power. Often, there is nothing that can be done but battle against the incursion until the Warp rift runs its course. Battles fought against a daemonic incursion are utterly different from those against mortal foes, for defensive structures and garrisons have little to no effect.