American counterterrorism agencies are quietly monitoring suspected sleeper cells on U.S. soil in the wake of joint U.S.–Israel strikes on Iran, stepping up surveillance amid heightened fears of possible retaliation from Iran-linked operatives or sympathizers. Federal and local law enforcement have also boosted on-the-ground security in major U.S. cities as part of a precautionary posture, even though no specific, credible threats have been publicly identified. The move comes...