For years, China kept buying Iran's oil. No matter what sanctions the U.S. piled on.
That lifeline is now drying up fast.
Iran sells most of its crude to China. It ships it at a discount on a fleet of old tankers.
In February that flow ran about 1.8 million barrels a day. By May it had crashed to about 160,000.
That's a drop of more than 90%. Most of those buyers sit in the emerging market China leans on to soak up cheap oil.
"The Iran-to-China oil trade is facing its biggest test yet," said John Driscoll, an analyst with four decades in oil.
Oil is Iran's main source of cash. So a fall this steep squeezes the whole country.
When the buyer who never walked away starts walking, that's a real problem for Tehran. We unpack stories like this in Market Briefs - five minutes a morning, with a free masterclass to sharpen your investing when you join.
First, China's buyers are pulling back. Its small private refiners are called teapots.
They buy about 90% of Iran's crude. But they've been losing money and slowing down.
Fresh U.S. sanctions have spooked them too. Beijing once told them to keep making fuel at all costs.
That order has now been eased. Energy Aspects expects these refiners to cut output by another 200,000 barrels a day in June.
China's own fuel tanks are full. Its refiners don't need to run hard.
The U.S. also hit a giant Chinese refiner, Hengli, with sanctions. That made other buyers even more careful.
Second, there's the blockade. The U.S. Navy has cut Iran's exports so tightly that no Iranian oil left through the Strait of Hormuz this month, by one firm's count.
Decades of sanctions never fully stopped Iran's oil. A wall of warships is the difference between a locked door and a "no entry" sign.
One you can ignore. One you can't.
China is not desperate for Iran's barrels. "China has access to a lot of other oil," one veteran analyst said.
He added that "there is no pressure at the moment." Cheaper Russian oil is filling some of the gap.
The squeeze is landing hardest at home. Oil output in Iran's economy fell 19% in a single month.
About 132 million barrels now sit on tankers at sea. At least 57 million wait off China and nearby waters.
Tehran has fewer places to turn. Sanctions block most Western buyers, which leaves Iran with crude it can't easily sell.
Earlier windfalls from high prices softened the blow for Iran. That cushion is wearing thin.
Output is down, and buyers keep backing away.
China holds the upper hand in any talks. It can afford to wait Iran out.
Want to know which headlines actually move markets? Join 350,000+ investors reading Market Briefs and get a 45-minute investing masterclass at no cost.
