It's time for the United States to hold Iran accountable.

President Trump, who has never made a secret of his hostility toward Iran, called recently for a grand regional strategy among Sunni nations to isolate the country. But Tehran received that threat with surprising equanimity because, in practice, the Trump administration has shown a willingness to do business with the country.

On the surface, it looked as if there was a lot of bad news recently for the Islamic Republic. At the recent Arab-American summit meeting in Saudi Arabia, Mr. Trump was the guest of the Saudi king, Salman bin Abdulaziz al Saud, a sworn enemy of Iran, and the countries signed a record-breaking $110 billion arms deal.

“Until the Iranian regime is willing to be a partner for peace,” Mr. Trump said at the meeting, “all nations of conscience must work together to isolate Iran, deny it funding for terrorism.”

In what seemed to be a response to the arms deal, Iran’s supreme leader, Ayatollah Ali Khamenei, said on Saturday that such purchases would lead to nothing.

“These fools think by spending money they can win the friendship of the enemies of Islam,” Mr. Khamenei said in remarks posted to his personal website. “They are like dairy cows. They will be milked, and when they are out of milk, they will be slaughtered.”

Tough talk from both sides, but back in Iran, they are awaiting the delivery of a fleet of American-made Boeing airliners, the result of two deals worth $22 billion for the United States company. The most recent contract between the plane maker and the Iranian airline Iran Aseman was signed two months after President Trump was sworn into office.

Mr. Trump, whose America First campaign was based in part on the promise of reviving industrial employment, was apparently not eager to kill an order estimated to create 18,000 jobs.

During the presidential campaign, Mr. Trump heaped scorn on the nuclear agreement with Iran, calling it “the worst deal ever.” But in April and May he quietly signed crucial waivers of certain sanctions that allow the deal to remain in place and let Iran conduct international business and gain access to funds long frozen by the United States.

Further evidence that the Trump administration is willing to engage with the Islamic Republic came during Secretary of State Rex W. Tillerson’s news conference in Riyadh, which followed the president’s hard-line speech. What if Iran’s foreign minister, Mohammad Javad Zarif, happened to telephone him? Mr. Tillerson said he would take the call.

“In terms of whether I’d ever pick the phone up, I’ve never shut off the phone to anyone that wants to talk or have a productive conversation,” Mr. Tillerson said.

The Trump administration appears to have grasped an important point about Iran: The very thing that the administration complains and worries about — Iran’s expanding influence in the region — makes it imperative that the two countries maintain at least a working relationship.

The United States will have a hard time solving problems in the Mideast without Tehran’s cooperation: in Lebanon, where it backs the Shiite militant group Hezbollah; in Syria, where it is propping up the government of President Bashar al-Assad; in Iraq, where it supports the government of Prime Minister Haider al-Abadi and trains powerful Shiite militias; and in Yemen, where to some extent, it is backing the Houthi rebels against the government.

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