How do you know when there’s a real crisis in U.S.-Israel relations? It’s when the president of the United States convenes a nuclear security summit to curb Iran’s nuclear ambitions, and the Israeli prime minister declines the invitation.

Benjamin Netanyahu, the Israeli prime minister, has made Iran’s nuclear threat to Israel’s existence the central organizing principle of his second term. Yet at the nuclear summit in Washington last week, President Obama was the one to do the heavy lifting, persuading China to join in a new round of U.N. sanctions against Iran.

Netanyahu explained that his presence at the summit would have prompted some leaders to focus attention on Israel’s nuclear program. But one suspects the real reason for his conspicuous absence was that he does not have an answer to President Obama’s demand that he freeze new building announcements in East Jerusalem for a few months to give peace negotiations with the Palestinians a chance to take off.

That an issue of as much strategic import to Israel and the United States as Iran could be subordinated to the demands of Netanyahu’s right-wing government underscores the growing divide between the Obama administration and the Netanyahu government.

The president views curbing Iran’s nuclear program and resolving the Israeli-Palestinian conflict as two sides of the same coin. In order to isolate and pressure Iran, he believes he needs to unite Israelis and Arabs with the rest of the world in a grand international anti-Iranian coalition.

The common threat is there — Arab leaders are at least as concerned as Netanyahu about Iran. But the inability to make progress on the Palestinian issue enables Iran’s leaders to play to the Arab street, claiming they are the real supporters of the Palestinian cause through sponsorship of violence and terrorism and threats to destroy Israel. The tension also gives Iran the opportunity to use Hamas and Hezbollah proxies to provoke conflict with Israel, with Mahmoud Ahmadinejad seen as the hero.

Netanyahu rejects these linkages. He argues that resolving the Palestinian problem won’t change Iran’s intentions or placate the Arabs; that it is Israel’s use of force that deters Hamas and Hezbollah; and that the split with the United States over building in East Jerusalem only encourages Tehran to believe that Obama will restrain Israel from striking Iran’s nuclear facilities.

Whoever is right, there is no denying the reality of a fundamental disagreement, one that has poisoned relations between the American and Israeli leaders.

At the heart of this disagreement lies a dramatic change in the way Washington perceives its own stake in the game. It actually began three years ago when Secretary of State Condoleezza Rice declared in a speech in Jerusalem that U.S. “strategic interests” were at stake in resolving the Israeli-Palestinian conflict — a judgment reiterated by Obama last week when he said resolving the Arab-Israeli conflict is a “vital national security interest” for the United States.

In other words, this is no longer just about helping a special ally resolve a debilitating problem. With 200,000 American troops committed to two wars in the greater Middle East and the U.S. president leading a major international effort to block Iran’s nuclear program, resolving the Israeli-Palestinian conflict has become a U.S. strategic imperative.

Ironically, as the U.S. position has evolved in this direction, Israeli attitudes have evolved in another. To many Israelis, especially those in Netanyahu’s right-wing coalition, peace with a divided Palestinian polity seems neither realistic nor particularly desirable.

Given Israel’s dependence on the United States to counter the threat from Iran and to prevent its own international isolation, an Israeli prime minister would surely want to bridge the growing divide. Yet the shift in American perceptions seems to have gone unnoticed in Jerusalem. Hence Netanyahu’s surprise when what he saw as merely a matter of a poorly timed announcement during Vice President Biden’s visit drew a stinging rebuke from Washington.

For Obama, however, Netanyahu’s apology doesn’t begin to address the real problem. His envoy, George Mitchell, had been struggling for nine months to launch Israeli-Palestinian negotiations. The day before Biden’s visit, Mitchell had announced agreement with Netanyahu and the Palestinian leader, Mahmoud Abbas, to commence “proximity talks.” The East Jerusalem building announcement came the next day, rendering those negotiations over before they had even started.

From Obama’s perspective, a zoning decision in an obscure Jerusalem suburb had dealt the United States a strategic setback. Deferring building announcements and other provocative actions in East Jerusalem thus became the litmus test of Netanyahu’s commitment to the common cause of curbing Iran’s nuclear enthusiasm.

As he studies his options, Netanyahu would do well to reflect on the decisions taken by two earlier prime ministers from his Likud Party — Menahem Begin and Ariel Sharon.

Begin gave up all of Sinai for a peace deal with Egypt that avoided a fight with Jimmy Carter over a Palestinian homeland. Sharon believed that the best way to survive politically was to allow no daylight to show between him and the president of the United States. That led him to propose full Israeli withdrawal from Gaza in order to head off what he foresaw as inevitable friction with the United States over the West Bank and Jerusalem. Both Sharon and Begin were excoriated by their right wings.

Or there’s the example of Yitzhak Rabin, Israel’s greatest strategic thinker. He believed that the best way for Israel to counter the threat from Iran, in the Middle East’s “outer circle,” was to make peace with the Arab “inner circle.” That led him to offer his hand in peace to the PLO’s Yasser Arafat, and to offer a full Israeli withdrawal from the Golan Heights to Syria’s Hafez al-Assad.

Today, nothing could better help Obama to isolate Iran than for Netanyahu to offer to cede the Golan, as four other Israeli prime ministers have, in exchange for peace with Syria, which serves as the conduit for Tehran’s troublemaking in the Arab-Israeli arena.

The shift in America’s Middle East interests means that Netanyahu must make a choice: take on the president of the United States, or take on his right wing. If he continues to defer to those ministers in his cabinet who oppose peacemaking, the consequences for U.S.-Israel relations could be dire.