So they've been behind every major collapse since Jesus was born, and Gotham City is next: They plan to release a special gas that will make everyone really scared and start punching each other to death, like a Black Friday sale -- but not on a Friday! The terror.
We've been taught that empires that grow too big, abusive, or arrogant are naturally predisposed to failure. That's why the Soviet Union collapsed, and that's why the colonists rebelled against Great Britain. Corruption, inequality, and bureaucratic bloat eat away at complacent civilizations from the inside, like a political chest-burster. But in Batman's world, the fate of nations are actually decided by a group of self-righteous ninjas sneaking around with sickly rats in their pockets.
The League of Shadows is eventually defeated because they underestimate the strength and drive of their supposedly soft enemies. Maybe Rome and 17th-century London were actually ideal civilizations that, if given time, could have pushed the planet into a Golden Age. But instead, the League murdered millions and put us all back at square one because they decided that the roads sucked, or whatever.
Countless historians have attempted to understand the turmoil that led to major historical downfalls, and all of their work has been pointless. The final chapter of every book on Roman history should just end with: "And then a*****e ninjas burned everything down." What sort of lessons are the people in this universe actually able to learn from history? Make sure a handful of elitists living in the mountains of Bhutan dig your civilization? That's a difficult lesson to apply to public policy.