Although Mr. Trump has since stopped his attacks on Qatar, presenting himself as a mediator, some senior advisers continue the fight. Breitbart News Network, which until recently was run by Mr. Trump’s onetime ideological firebomber Stephen K. Bannon, has published dozens of articles attacking Qatar as a rogue ally.

Is Qatar soft on terrorism? Some of the charges are red herrings, American officials say. Tamim cut funding to most extremist militias in Syria and Islamist groups in Libya in 2015, at the urging of the Obama administration. His cordial ties with Iran are a matter of necessity because the two countries share the giant gas field that is the source of Qatar’s wealth.

Where Qatar does have a case to answer, officials say, is in its treatment of Qatari citizens accused of financing terrorist groups like Al Qaeda. Trials of accused financiers, when they take place, occur in secret, making it hard to know what punishment, if any, is imposed.

Abd al-Rahman al-Nuaymi, a former university professor and financier who has been designated a terrorist by the United Nations and the United States, was tried secretly in 2015 and acquitted. He later lived openly in Doha, albeit with restrictions on his banking and ability to travel, said a former United States treasury official who was briefed on the case. A senior Qatari official said the prosecution had received new evidence in the case, and Mr. Nuaymi had been arrested pending a second trial.

But similar charges can be laid at the feet of Qatar’s foes. The Saudis have long been accused of exporting radical Islam across the world through hard-line madrassas. Iran’s biggest trading partner in the region is not Qatar but Dubai. Human rights abuses and press freedom restrictions are far harsher in the Emirates.

For Qatar’s supporters, the hypocrisy reveals what they say is the boycott’s true goal: to cut down or take out Qatar’s youthful emir, the royal who refuses to go along to get along.