Tolkien got this. Defeating Sauron didn't end the story — the Shire still had to be scoured. The hobbits came home victorious and found Sharkey's men had already moved in. Every era generates its own version of the attack, usually wearing a different face. The threat after the great battle isn't the Dark Lord — it's the slow administrative capture you didn't see coming because you were celebrating. Eternal vigilance isn't pessimism. It's just knowing the map.