JERUSALEM — President Biden on Thursday issued one of the bluntest warnings to Tehran of his presidency, committing to Israel’s leaders that “we will not allow Iran to acquire a nuclear weapon,” but Israel’s caretaker prime minister, Yair Lapid, pushed even further, asking all democratic nations to vow to act if the Iranians continue “to develop their nuclear program.”
The distinction between Mr. Biden’s vow to stop a “weapon” and Mr. Lapid’s insistence on destroying Iran’s entire “program” was more than semantic: It goes to the heart of their countries’ differing approaches in dealing with Iran’s nuclear ambitions.
Even amid frequent and public affirmations of the close relationship between Israel and the United States, the differences over how to handle Iran remain stubborn. Several times on Thursday, members of Israel’s leadership publicly and privately urged that the United States develop a more credible military option to take out Iran’s nuclear facilities, as a way of convincing Tehran it must halt a rapidly accelerating program.
“If they continue to develop their nuclear program, the free world will use force,” Mr. Lapid said at the opening of a news conference in Jerusalem after the two leaders met as part of Mr. Biden’s four-day visit to the Middle East.
During those remarks, Mr. Biden listened attentively but never repeated that commitment. Instead, he stuck to talking about blocking Iran from obtaining a weapon — not a “program” that might be intended to develop one.
But even these long-running differences of strategy are shifting, amid cracks in Israel’s own consensus about how imminent and urgent a threat is posed by the Iranian nuclear program.
And on Thursday, those differences on Iran strategy were largely set aside on the first full day of Mr. Biden’s first trip to the Middle East as president, in a region where alliances and relationships have changed radically since he was last here as Barack Obama’s vice president.
On Friday he moves to the trip’s harder task: trying to revive the alliance with Saudi Arabia, amid sharp criticism, especially from the progressive wing of his own party, that he is rehabilitating a crown prince whom the C.I.A. believes was knowledgeable of, and perhaps complicit in, the murder of Jamal Khashoggi, an American-based dissident and columnist.
Mr. Biden’s mission in Jerusalem was to bolster and deepen the relationship with Israeli leaders while stepping around a roiling election for a new prime minister.
And Mr. Biden used Thursday’s news conference with Mr. Lapid to bolster the blossoming relationship between Israel and a handful of Arab states, including the creation of a joint air defense zone to protect against Iranian drones and missiles. Administration officials say that while they are pushing for full diplomatic relations between Saudi Arabia and Israel, they expect only incremental progress toward that goal on this trip.
But it is Mr. Biden’s own relationship with Saudi Arabia that looms largest over the second part of his visit. In the short news conference on Thursday, Mr. Biden was pressed directly on whether he would raise the case of the killing of Mr. Khashoggi when he meets with Saudi leaders on Friday. Crown Prince Mohammed bin Salman is accused of directly approving the brutal 2018 killing in Istanbul of Mr. Khashoggi, a columnist for The Washington Post who was living in the United States.
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Mr. Biden said on Thursday that his views on the killing were well-known, but he stopped short of saying whether he would specifically raise the dissident’s name during his meeting with Prince Mohammed.
“My views on Khashoggi have been absolutely, positively clear,” Mr. Biden said, adding that he had never hesitated to speak openly to allies and adversaries about human rights. But with the American leader scheduled to fly directly from Israel to Jeddah on Friday — a flight that itself says much about the changed environment in the Middle East — administration officials were still debating how, if at all, he should raise the case in public comments on Saudi soil.
In other cases, recently including Cuba and Venezuela, Mr. Biden has stressed that his administration is making democracy and respect for human rights the paramount consideration for dealing with other nations’ leaders. But on Thursday in Jerusalem, he said “the reason I am going to Saudi Arabia is to promote U.S. interests.” Those include getting the kingdom to pump more oil from its somewhat modest spare capacity.
Mr. Biden was clearly in his element all day in Jerusalem. These were the kinds of trips he loved as chairman of the Senate Foreign Relations Committee and later as vice president.
Mr. Biden celebrated the signing of a new “Jerusalem Declaration,” a restatement of the solidity of the alliance between the two countries, of American commitments not to permit Iran to obtain a weapon, and of Israel’s rapprochement with many of the Arab adversaries that had tried to undercut the creation of a Jewish state.
While little in the declaration was new, the fact that it stated so boldly the broad outlines of the relationship — signed by a Democratic president who many in Israel viewed with suspicion, and by an acting Israeli prime minister seeking to make his role permanent — dominated much of the public discussion in Israel.
On Thursday Mr. Biden was awarded the Israeli Presidential Medal of Honor, and, borrowing from the Torah, he called Israel “a nation that will never dwell alone, because as long as there’s the United States you will never be alone.”
Presiding over the medal ceremony, Isaac Herzog, Israel’s president, whose policymaking role is limited, said government officials had found a record of Mr. Biden’s first trip to Israel as a young senator in 1973. During that visit, Mr. Herzog read, Mr. Biden “was carried away by his enthusiasm,” a description that seemed no less apt nearly a half-century later.
Later on Thursday, Mr. Biden attended the opening of the Maccabiah Games, a quadrennial international Jewish sporting competition.
Mr. Biden spent much of his trip touting common projects between Israel and the United States, starting with the Iron Dome rocket-interception system and a new system named Iron Beam, still a prototype, that uses lasers. Mr. Biden watched a demonstration as soon as he landed in Israel, setting the tone for the rest of his trip.
“These technologies and advances are critical,” Mr. Biden said. “Every rocket that is intercepted is a potential life, maybe more, that is saved.”
His commitment to stopping Iran from actually acquiring a nuclear weapon was not new — George W. Bush, Barack Obama and Donald J. Trump had all made similar vows — but his tougher-sounding language was unusually explicit, including the promise to use military force if necessary. (Mr. Obama, for example, would avoid outright threats and talk instead about employing “all instruments” of American power — financial, diplomatic and military.)
Israel has pursued a policy for several years now of repeatedly blowing up facilities and assassinating leaders of the nuclear program in an effort to slow Iran’s ability to produce nuclear fuel. That covert program has sped up in the past year, and Israeli officials sometimes have called it “mowing the lawn,” a recognition that as fast as they blow up elements of the program, the Iranians try to rebuild.
The United States is pursuing a different track, trying to revive the diplomatic accord with Iran, now seven years old, that Mr. Trump abandoned. That deal required Iran to ship 97 percent of its nuclear fuel out of the country, and many Israeli military and intelligence officials say they now think Mr. Trump’s move to abandon the agreement failed, allowing Iran to resume and accelerate its nuclear enrichment program. Mr. Biden reaffirmed on Thursday his belief that diplomacy offers the only hope of a lasting solution.
For Mr. Lapid, taking an uncompromising stance on Iran in the presence of the American president may have been a political imperative heading into November elections, when he hopes to convert his caretaker status to a full term as prime minister.
For years, Mr. Lapid has endeavored to avoid letting Benjamin Netanyahu, Israel’s former prime minister, outflank him to the right on security issues, though on Thursday Mr. Netanyahu, after meeting Mr. Biden, said he told the president “a credible offensive military option is needed.”
But Mr. Lapid’s challenge for Mr. Biden was softened by exuberantly friendly body language, and the session had none of the bristling tension that sometimes marked Mr. Netanyahu’s meetings with Mr. Biden when he was vice president. In private, some Israeli officials say they are focusing more on Iran’s support for terror groups in the Middle East, and that they think they would have sufficient warning if Iran actually moved to build a weapon.
Mr. Biden did not seem to take offense at Mr. Lapid’s public disagreement. Indeed, when Mr. Lapid finished speaking at the news conference, Mr. Biden offered praise. “An eloquent statement,” he said.
Patrick Kingsley, Peter Baker, Isabel Kershner and Hiba Yazbek contributed reporting from Jerusalem.