The rescue had unfolded with near-perfect precision. Under cover of darkness, U.S. commandos slipped deep into Iran, undetected, scaled a 2,100-meter ridge and pulled a stranded American weapons specialist to safety, moving him toward a secret rendezvous point before dawn on Sunday.

Then everything stopped.

Two MC-130 aircraft that had ferried some of the roughly 100 special operations forces into rugged terrain south of Tehran suffered a mechanical failure and could not take off, a U.S. official said, speaking on condition of anonymity.