Folly requires that a better alternative was visible at the time, not just in hindsight

Tuchman is precise about what she means by folly, distinguishing it sharply from decisions that simply turned out badly given the information available at the time. Her four case studies are chosen specifically because, in each, contemporaries raised clear warnings and viable alternative courses of action existed and were articulated, sometimes forcefully, by advisers within the decision-making circle itself, yet were dismissed or ignored.

This distinction matters because it's what makes the pattern instructive rather than merely tragic: a leader who makes a reasonable bet that fails isn't guilty of folly in her sense, but a leader who is warned, has a workable alternative available, and proceeds anyway toward a foreseeable disaster is engaged in something Tuchman considers a recurring, diagnosable failure of governance.

Takeaway: the most instructive failures aren't the ones no one saw coming, they're the ones everyone saw coming except the person in charge.