DUBAI, Aug. 17 -- Every tanker captain transiting the Strait of Hormuz on Monday will be operating in a legal vacuum. The memorandum of understanding that ended three months of war between the United States and Iran expires that day, and neither side has made any move to replace it.

The agreement, signed in Islamabad on June 17 by President Donald Trump and Iranian President Masoud Pezeshkian following Pakistan-hosted mediation, was supposed to hold for sixty days while negotiators produced a final settlement. No settlement was produced. Both governments accuse the other of violating the terms before the ink was dry. The sixty days run out Monday.

Iran's Foreign Minister Abbas Araghchi was unambiguous about where Tehran stands. "We have...