A handout picture released on March 8, 2015 by the official website of the Centre for Preserving and Publishing the Works shows Iran's supreme leader Ayatollah Ali Khamenei. (Ho -/AFP/Getty Images)

THE OBAMA administration is seeking to assure U.S. allies and congressional skeptics that the nuclear accord it is contemplating with Iran will not lead to a broader detente with the Islamic republic. “We are not seeking a grand bargain,” Secretary of State John F. Kerry declared last week during a visit to Riyadh he made with the explicit purpose of countering Saudi Arabian suspicions to the contrary. “We will not take our eye off of Iran’s other destabilizing actions in places like Syria, Lebanon, Iraq and the Arabian Peninsula.”

The political imperative behind this clarification is easily understood. In recent months, the notion that President Obama is prepared to scrap the 35-year-old U.S. policy of seeking to contain Iranian influence in the Middle East has been widely accepted by Arab and Israeli officials and U.S. commentators; opposition to such a reversal is one reason the prospective nuclear deal is generating bipartisan unease in Congress.

The president himself has provided much of the fuel for the speculation. According to news accounts, Mr. Obama has dispatched four private letters to the Iranian supreme leader, Ayatollah Ali Khamenei. The most recent one, in October, assured the ayatollah that the United States would not attack Iranian forces or those of its Syrian ally in operations against the Islamic State, according to the Wall Street Journal. Publicly, Mr. Obama said in an interview in December that he hoped a nuclear deal “would serve as the basis for us trying to improve relations over time”; if Iran agreed to the accord, he added, it could become “a very successful regional power.”

Such statements have understandably alarmed Mideast leaders at a time when Tehran is engaged in what Israeli Prime Minister Benjamin Netanyahu called a “march of conquest, subjugation and terror.” A relaxation of U.S. efforts to resist this bid for regional hegemony would be a strategic calamity for Israel and the Persian Gulf states. Saudi Foreign Minister Saud al-Faisal said after speaking to Mr. Kerry that it was “really the main concern of the Gulf Cooperation Council.”

Unfortunately, the administration’s assurances are at odds with its actions. While the nuclear negotiations have continued, Mr. Obama has refused to support military action against the Assad regime in Syria, in accord with his letter’s reported promise, and his administration has tacitly blessed an ongoing, Iranian-led offensive in Iraq’s Sunni heartland. It took no action to stop the ouster by an Iranian-backed militia of a pro-U.S. Yemeni regime. Nor has it reacted to Iran’s deployment of thousands of Shiite fighters to southern Syria, near the Israeli-occupied Golan Heights.

That record raises the question of what the administration’s response will be to further Iranian adventurism following a nuclear deal. Will it help its allies fight back, or will it restrain itself in the interest of preventing a rupture of the nuclear accord and in order to “improve relations over time”? Mr. Obama argued last week that if Iran obtained nuclear weapons it “would make it far more dangerous and would give it scope for even greater action in the region.” That’s clearly true; the worry is that his current policies, combined with the lifting of sanctions, could have the same result.