For nearly three quarters of a century, global order rested on a presumption so deeply embedded it scarcely required articulation: When deterrence faltered, the United States would act. Not always swiftly, not always cleanly, but decisively enough to restore equilibrium. This presumption — the quiet engine of Pax Americana — restrained revisionist powers, reassured allies and allowed secondary powers to flourish beneath Washington’s security umbrella.

That presumption no longer holds.

Not because American capability has vanished. It has not. But because American reliability — the expectation that Washington will act predictably in defense of its partners — has weakened in a way that matters just as much as capability itself.