The Strait of Hormuz, the world’s most important oil chokepoint, is notoriously tiny. On Thursday, Japan was stuck right in the middle of it.
Early in the day, two tankers were reportedly attacked in the Gulf of Oman, which leads into the Strait of Hormuz: a tiny water passage that links the energy giants in the Persian Gulf to the energy-hungry Asian markets. Iran is on one side of the Strait; Oman’s on the other. The strait’s importance can’t be overstated: about 40% of the world’s physical crude traded daily passes through it, according to Rystad Energy.
Both the tankers were bound for Japan, according to the Associated Press, with at least one registered to a Japanese shipping company. Thursday’s attack followed reports last month from Saudi Arabia that several oil tankers in the region had been sabotaged. (It’s unclear who’s responsible for any of the incidents.) The latest episode immediately heightened fears of an interruption to flows through the region—there are few other alternatives—particularly as tensions have been rising between U.S. and Iran over the revival of sanctions.
For Japan, the timing was particularly bad: Japanese Prime Minister Shinzo Abe was in Tehran this week, the first Japanese leader to visit in four decades. He was meeting with Iran President Hassan Rouhani, who was pressuring Japan—a firm ally of Washington—to sidestep U.S. sanctions and resume buying Iranian oil.
Japan has not been doing so, choosing instead to comply with the revival of U.S. sanctions and the withdrawal of a waiver that had allowed Japan to continue importing Iranian oil until the beginning of May.
That has put the country in an awkward position. Japan is the fourth largest consumer of oil in the world, coming only after the U.S., China, and India, according to the U.S. Energy Information Administration (EIA). It also relies almost entirely on imports to get its oil, making it one of the world’s largest importers. And for the last several years, both the U.S. and Iran have been suppliers.
Japan is one of the U.S.’s largest export markets for crude and petroleum products, according to the EIA, a trend that began in 2015 as the shale boom took off and hit a historical high in November of last year.
But on the import side, U.S. product is still only a fraction of Japanese demand.
Taking crude alone—only one part of total petroleum exports—U.S. product made up only 2% of total crude imports into Japan in April, according to data from Japan’s Ministry of Economy, Trade and Industry (METI). By contrast, Japan’s Iranian crude imports in April, the final month Japan had its waiver, were more than four times the size of the American imports.
Even still, there’s a larger risk here for Japan, and that’s in the Strait itself. Just four countries—Saudi Arabia, Kuwait, Qatar and the U.A.E.—make up nearly 80% of Japan’s crude imports.
How does all that oil get to Japan? Through the Strait of Hormuz.
More must-read stories from Fortune:
—Manufacturers are leaving China—for reasons beyond the trade war
—Cruises to Cuba are banned, but the ships sail on
—This is the one subject in the U.K. that’s as toxic as Brexit
—German security chiefs say Alexa should provide evidence in court
—Listen to our new audio briefing, Fortune 500 Daily
Keep up with the world’s most powerful women with Fortune‘s Broadsheet newsletter