The European plan to get around the American sanctions reflects the depth of allied outrage over Mr. Trump. Whether it can work is another story. Major European companies, like the French oil giant Total, fearful of being shut out of the American market, have already cut ties with Iran. And some doubt that the mechanism will ever be put in place.

Some analysts say it could eventually benefit competitors eager to replace the dollar as the world’s dominant currency and do long-term damage to the United States.

Mr. Trump faces other trouble as well. No matter how painful the reimposed sanctions are, they won’t be as effective as when the major powers were united in their enforcement. China, India and Japan have signaled a willingness to continue at least some purchases from Iran.

As history shows, sanctions are most effective when they are applied judiciously and with broad international support. Conversely, the United States and its partners must be willing to remove sanctions when the target country changes its behavior.

President Barack Obama saw the nuclear deal as a way to prevent Iran from producing a nuclear weapon, to ease the hostility that has dominated relations since the 1979 Islamic Revolution and to appeal to a new generation of pro-Western Iranians.

Many hoped a less confrontational approach would eventually open the door to solving other problems, like Iran’s ballistic missile program; hostage-taking; threats to Israel; and military involvement in Syria, Yemen and Lebanon.

That not only didn’t happen, but in many ways Iran’s malign regional behavior has grown worse, a bad call by the country’s leaders.