DUBAI, Aug. 25 -- For the third time in eight days, a ballistic missile launched from Yemen has struck a vessel in the Red Sea. The attack on a Saudi oil tanker on Monday, 63 nautical miles west of Yanbu, set fire to the vessel's main deck and sent maritime insurers racing to revise risk maps for Saudi Arabia's entire western coastline. All crew survived. The question of how the kingdom defends its most critical bypass corridor did not.

Yemen's Houthi forces confirmed the strike. "The strike was accurate and direct, resulting in a fire breaking out aboard the vessel," the group said in a statement. UK Maritime Trade Operations confirmed the incident through its maritime alert system, reporting that all crew had been accounted for and the...