The underlying mechanic is always the same: You might be able to deny ground from the air to some extent, but you can’t hold it without boots on the ground. Morale bombing has been tried aplenty and has yet to show convincing effect. And both aircraft and grunts are vulnerable to all kinds of unpleasantness from the ground.

For a defender, the primary objective is making it so costly to sustain the attack that the attacker either can’t afford to or decides it’s not worth it. Asymmetric warfare has shown potential to be quite efficient at that task. The Taliban are one example, the Vietcong another and I really hope we don’t have to find out if Denmark belongs on that list.