Soon after taking office last year, President Trump issued a directive laying out his “maximum economic pressure” policy against Iran. Yet Mr. Trump seems to have decided that even maximum pressure can be turned up to 11. He calls his new plan “Economic D-Day,” and it is meant to break the stalemate in the U.S. conflict with Iran.

On Monday, Treasury Secretary Scott Bessent announced new sanctions against 60 Iranian and Iran-linked entities, individuals and vessels, and demanded the closing of all branches of a major Iranian bank that operates in many countries. But his demands that other nations stop doing business with Iran underscore the difficulty in trying to impose sanctions on Iran’s economy to the point of collapse. Thoroughly enforcing sanctions is notoriously hard, and in Iran’s case would require messy run-ins with allies and powerful rivals capable of retaliation, most notably China.

Since Iran’s 1979 Islamic Revolution, the U.S. has placed sanctions on thousands of specific Iranian officials and businesses. “We’ve done all the low-hanging, medium-hanging and high-hanging fruit,” said Alan Eyre, a former U.S. diplomat and Iran expert. “We’ve cut down the whole sanctions tree. There’s nothing left.”

All that provides reason for skepticism about Mr. Trump’s latest plan. Iran’s economy took a toll as oil exports plunged, but nearly 50 years of American sanctions have done little to change Iran’s behavior. U.S. officials still struggle to strangle Iran’s economy despite the mountain of sanctions Washington has heaped on the country.