HOW WILL Iran respond to the U.S.-Russian initiative to control and destroy Syria’s chemical weapons? President Obama took a rosy view in an interview broadcast Sunday, telling ABC News that “the Iranians recognize they shouldn’t draw a lesson that . . . we won’t strike Iran” to stop its nuclear program from the fact that force was not used against Syria. On the contrary, the president asserted, Tehran “should draw from this . . . that there is the potential of resolving these issues diplomatically.”

Set aside for the moment Mr. Obama’s clumsy claim that “the Iranians understand that the nuclear issue is a far larger issue for us that the chemical weapons issue,” made just a few days after he said in a televised address that Syria’s chemical weapons use was “a crime against humanity” that puts at stake “our ideals and principles, as well as our national security.” Is he right to think that his head-snapping shift from proposing military strikes against Syria to embracing diplomacy will make Iran more, rather than less, inclined to bargain seriously over its enrichment of uranium?

In fact, the signals are mixed. Some Iran-watchers believe that recently elected President Hassan Rouhani is exploring the possibility of rapproachment with the United States. He and Mr. Obama have exchanged letters, and U.S. and Iranian officials are reported to be exploring the possibility of direct talks between the two governments for the first time since 1979. Mr. Obama and Secretary of State John F. Kerry have hinted at possible Iranian participation in a peace settlement in Syria — something the Obama administration previously ruled out. There are reports that Mr. Rouhani is preparing to unveil a proposal to limit its uranium enrichment in exchange for the lifting of sanctions. If so, Mr. Obama’s acceptance of a diplomatic path in Syria could encourage that initiative.

At the same time, the president’s ad hoc maneuvering could offer encouragement to Iranian hard-liners. His sudden decision to postpone an attack in favor of consultations with Congress — and the stiff resistance he then encountered — raised questions throughout the Mideast about U.S. resolve. Mr. Obama’s repeated declarations that he has no intention of intervening in Syria’s civil war could be taken by Tehran as meaning that it can continue bolstering the regime of Bashar al-Assad with impunity. Why participate in a negotiated settlement if there is no chance that the United States will tip the military balance against Iran’s client?

In the end, Iran’s response to the emerging Syria deal — like that of its adversaries Saudi Arabia and Israel — will depend on whether and how it is implemented. If the Assad regime seeks to conceal part of its arsenal, impede inspections or drag out the agreed timetable for action, a failure by the Obama administration to respond forcefully would send a fateful message of weakness to Tehran. It would also probably convince Israel that it can no longer be constrained by the United States from taking its own military action against Iranian nuclear facilities. It is consequentially crucial that Mr. Obama make clear that he is prepared to carry out the military strike he had planned against the Syrian regime if Damascus does not comply — beginning with this week’s required declaration of its chemical weapons and facilities.