Definition
Irreversibility is the political principle that some decisions, once made, cannot be undone within any politically relevant time horizon. A bridge built across an estuary cannot be unbuilt without billions of dollars and a generation of political will. A bond covenant signed today binds revenue for forty years. A neighborhood demolished cannot be reassembled. The asymmetry between the difficulty of building and the difficulty of unbuilding is the structural fact on which much of The Power Broker turns.
Robert Moses understood this principle earlier and more deeply than his opponents. His strategic technique was to convert a contested decision into an irreversible one as quickly as possible — by pouring foundations before opposition could mobilize, by selling bonds before legislatures could reconsider, by signing contracts before mayors could object.
Why it matters
How it works
A decision becomes irreversible by combining three commitments. First, large fixed capital is sunk — physical structures, signed contracts, paid-out grants — that cannot be recovered if the project is canceled. Second, a coalition of beneficiaries forms — contractors, employees, users — who will defend the project politically. Third, the surrounding land use, traffic pattern, or institutional arrangement reorganizes around the new fact, so that reversal would require redoing all of that adaptation too.
The asymmetry creates two strategic implications. For the builder, speed is decisive: get the foundation poured and the bonds sold before deliberation can catch up. For the opposition, the only effective intervention is before the irreversibility threshold; once the concrete is set, even successful political campaigns can only modify, not undo. This is the reason Caro spends so much of the book on the small choices made before a project became inevitable — those are the decisions that mattered, even though they look small in retrospect.