Tehran, Sept. 28 -- Iran framed the impending return of United Nations sanctions as a political ritual rather than an existential turning point, casting European capitals and Washington as the authors of an avoidable failure. With the clock ticking toward the formal reimposition at 8 p.m. EDT on Saturday, officials in Tehran pledged a firm but calibrated reply and recalled ambassadors from London, Paris, and Berlin for consultations, a hardening of posture that stops short of detonating the nonproliferation framework that still underpins the file.

The legal process traces to the mechanism attached to the 2015 nuclear deal in Security Council Resolution 2231, a boomerang embedded so that any sustained dispute would snap measures back into...