(John Moore/Getty Images)

Anticipation has been building for weeks that this year's United Nations General Assembly would present an historic opportunity for President Obama to reach out to his new counterpart in Iran, the recently elected Hassan Rouhani, who has openly espoused detente with the West. The excitement has built to such heights that even the possibility of a simple handshake between the two leaders – which would be the first between an American and an Iranian leader since the 1979 Islamic revolution – has generated international headlines.

But when Obama took the lectern at the General Assembly on Tuesday, his tone toward Iran was far more measured and skeptical than Rouhani's has been toward than United States, or than even Obama's own language in prior speeches. Despite recent signals from the Obama administration that they are willing to consider reciprocating Iran's outreach, as well as Obama's years of addresses offering Iran a peaceful resolution to their disagreements, he took a tougher-than-expected line on the Islamic Republic.

Before he even mentioned Iran by name, much less the chance for detente, Obama stressed his willingness to use military force to prevent any Middle Eastern state from acquiring nuclear weapons – an clear threat to strike Iran. And he pledged to "confront external aggression against our allies and partners, as we did in the Gulf War," reiterating his promise to defend Israel.

Obama's 45-minuted U.N. address included just five short paragraphs on the apparent opportunity for detente with Iran. Most tellingly, he introduced that section by saying that "Iran's pursuit of nuclear weapons" would be one of two issues on which "America's diplomatic efforts will focus." While he suggested that resolving the nuclear issue could "serve as a major step down a long road toward a different relationship," he repeatedly stated that the United States wanted first and foremost to prevent Iran from developing nuclear weapons.

Obama seemed to be stating that the United States would pursue engagement with Iran not for the sake of engagement or even detente, but as a means to an end: preventing a nuclear-armed Iran. That was a tougher message, and a narrower aim, than Obama has expressed in his annual Nowruz message, in which he's used the traditional Persian new year to offer peaceful coexistence with Iran.

That Obama would harden his stance toward Iran, at precisely the moment when Tehran seems most receptive to his entreaties, may seem surprising on the surface. But U.S.-Iran engagement is shifting from theoretical to actual this week. And that means the United States is a little less worried about enticing Tehran to the negotiating table, and a little more preoccupied with keeping their Iranian counterparts honest.

But the toughening stance on Iran, like the decision to privilege the nuclear issue far above detente, seemed to nod to growing concerns from Israel. Prime Minister Benjamin Netanyahu and other Israeli officials have warned that they believe Rouhani's talk of peace is a ploy, meant to buy time for Iranian nuclear progress and to fool the West into backing down. A number of Iran hawks within Washington have warned the same.

Israel does not have veto power over U.S.-Iran engagement, exactly, but it does have significant influence – and sympathetic-minded legislators in Congress could have the power to block Obama from any deals with Tehran. It's unlikely that either Netanyahu or D.C.-based Iran hawks would ever fully endorse a grand bargain between the United States and Iran, of the sorts that Iranian diplomats proposed in 2003. But their level of skepticism, and thus their level of opposition, could play an important role in shaping the course of any negotiations.

This may go a long way toward explaining Obama's shift in tone. He likely sees a need to reassure Israeli officials and U.S. Iran hawks that any direct engagement with Tehran, no matter how chummy it may look, is just another way of securing their mutual goal of a nuclear disarmed Iran. That means elevating Israeli priorities a bit; it also means addressing, even if only implicitly, Israeli concerns that the Obama administration will be duped by an Iranian ploy to buy time for enrichment.

The United States has been an indirect mediator of sorts between Israel and Iran for years. Obviously one of those countries is a close U.S. ally and the other very much is not. And the United States has worked closely with Israel to curb the Iranian uranium enrichment that concerns them both. But the United States has also sought to rein in Israeli hawkishness on Iran, including threats of a unilateral military strike against suspected enrichment sites. Any U.S. policy toward Iran, whether confrontational or diplomatic, requires a degree of managing Israel. That's just how international relations works, whether it's between allies or adversaries.

It's likely that U.S. officials elsewhere are quietly fostering engagement with Rouhani's new government, and Secretary of State John Kerry is to meet this week with Iranian Foreign Minister Javad Zarif. But if Obama is going to pick up any degree of Israeli support or even acquiescence for a deal with Iran, Obama has to address Isreal's concerns, taking them on board as his own. And that means a tougher and more skeptical line.