BAGHDAD — The driver was sweating as his white Kia pickup truck sped along a rain-slicked Baghdad highway toward a neighborhood bustling with open-air markets.

With every jolt and turn, his pulse quickened. Hidden in the truck’s chassis was 1,100 pounds of military-grade explosives that the Islamic State planned to use in an audacious attack on New Year’s Eve shoppers in the Iraqi capital.

A reckless driver on Iraq’s notoriously chaotic roads might clip him, accidentally setting off the bomb. A clash at one of Baghdad’s frequent checkpoints could escalate into gunfire, potentially igniting one hellish fireball.