In their first Oval Office meeting, in May 2009, President Obama and Israeli Prime Minister Benjamin Netanyahu smiled, shook hands and pledged to do everything possible to keep Iran from getting a nuclear weapon.
That united front — and their personal relationship — has been unraveling ever since.
Though the two leaders say their objective has not changed, they have tussled over Iran for six years because of their conflicting convictions, ambitions and assessments.
On Tuesday, their battle will be on display as never before.
In a show of defiance, Netanyahu will go before a joint session of Congress to ask U.S. lawmakers to prevent Obama from striking a deal that would limit — but not end — Iran's nuclear program. The Israeli leader says the agreement under discussion would put his country in peril.
Some say the collision was predictable.
"Their positions have always been irreconcilable," said retired Israeli Brig. Gen. Shlomo Brom, a scholar with the Institute for National Security Studies in Tel Aviv.
Obama's goal has been to resolve one of the West's foremost security worries by negotiating a deal that would curtail Iran's enrichment of uranium and other nuclear activities for at least a decade, under intensive scrutiny by the United Nations' nuclear watchdog agency, in exchange for easing sanctions on Iran's economy.
Working with Britain, France, Russia, China and Germany, U.S. negotiators are racing to complete the outline of a deal with Tehran by the end of March. If they succeed, they have a June 30 deadline to nail down the technical details for a comprehensive agreement.
Netanyahu wants Iran's nuclear program dismantled, not limited. He fears a 10- or 15-year deal would simply give Iran more time to cheat, or postpone the day when Tehran would have free rein to build a nuclear weapon — something it insists it has no intention of doing.
And he worries that once an agreement is signed, the U.S. and its allies won't have the political will to use military force, if necessary, to stop Iran from cheating.
"On such a critical topic, which could determine whether we exist or not, it is my duty to prevent this great danger to the state of Israel," Netanyahu said Wednesday during his reelection campaign.
Netanyahu, a twice-wounded former Israeli commando, made stopping Iran a core campaign issue when he entered politics 27 years ago. Many Israelis, including many conservative politicians, don't consider the threat as urgent as he does, analysts say.
Some Israeli critics charge he has picked a fight with Obama to bolster support from Israel's far right to help win reelection on March 17. But even Israelis who disagree with him acknowledge his sincerity.
"This isn't just about political considerations for him," said Brom, who is now a visiting fellow at the Center for American Progress, a liberal-leaning research and advocacy organization in Washington.
Netanyahu sees himself standing steadfast, like Winston Churchill warning of Nazi Germany before World War II, when other world powers didn't have the stomach for a fight. He has often compared Iran to Nazi Germany.
Netanyahu believes Israel must take a tougher line because it's so vulnerable.
Though Israel is the greatest military power in the region, it doesn't have the overwhelming military dominance of the United States. So it doesn't have as many options or as much time to respond if it discovers that Iran is secretly racing to build a bomb.
The United States first accused Iran of seeking nuclear weapons in 2002, during the Bush administration. Over the next few years, Iran repeatedly resisted U.N. inspections of its nuclear facilities and installed several thousand centrifuges, fast-spinning devices that enrich uranium.
At low levels, enriched uranium is used for energy reactors. When refined to high grades, it can be used as bomb fuel.
In December 2007, U.S. intelligence concluded that Iran had "halted its nuclear weapons program" in 2003, but "that it has the scientific, technical and industrial capacity eventually to produce nuclear weapons if it decides to do so."
That is still the official U.S. view. Once in office, the Obama administration decided it would be futile to try to persuade Iran to give up its enrichment program.
Netanyahu sharply disagreed, insisting that any enrichment put Iran on the nuclear "threshold" to build a bomb in a dangerously short time.
His worries grew in November 2013, when diplomats struck an interim deal with Iran to temporarily freeze parts of its nuclear program in exchange for easing some sanctions. That started the clock for Iran and the six powers to negotiate a final deal.
Netanyahu saw a double danger. The interim deal signaled for the first time that world powers might support Iran's enrichment program. The West also began to ease minor sanctions, backing off, in Netanyahu's view, at the moment of maximum leverage.
He has grown more critical in recent months as leaks from the closed-door negotiations suggested other concessions.
Western diplomats talked in 2013 about a deal lasting 20 or 30 years. Now they're reportedly considering one that would begin easing restrictions after 10 years and give Iran the leeway of any law-abiding nation in 15 years.
They insisted at the start that Iran could run no more than 1,500 rudimentary centrifuges. Now they're talking about allowing 6,500.
"There's a sense the concessions have mostly been on the U.S. side, while Iran has been consistent and steadfast," said Michael Singh, managing director of the Washington Institute for Near East Policy, a pro-Israel think tank in Washington.
A central worry for Netanyahu is enforcement. After a deal is signed, it may be very difficult to persuade nations that have resumed normal trade with oil-rich Iran to renew economic sanctions, or deploy military force, for what may appear technical violations.
"This is a very valid fear," said Jofi Joseph, a nonproliferation expert on the National Security Council staff early in the Obama administration.
Administration officials contend that they would accept only a deal that makes it impossible for Iran to stage a "breakout," a race to accumulate enough nuclear fuel for one bomb, in less than one year. That would give world powers ample time to intervene, they say.
And they argue that a negotiated deal is a far better outcome than a collapse of talks, which would leave Iran free to resume unrestrained enrichment and other nuclear activities. A Western military attack, the other option, would end any international oversight or monitoring, and might halt Iran's program for only a few years while starting a new Middle East war.
Netanyahu is unconvinced.
After Tuesday's speech, he may launch an open campaign to line up votes in Congress to strike down any deal with Iran.
Some critics warn that his high-stakes strategy could backfire, making it easier for the White House to build public support for its deal, especially among Democrats.
Netanyahu says he has no choice. "It's my sacred duty as the prime minister to make Israel's case," he said recently to an American pro-Israel group.