A nuclear deal with Iran would be a diplomatic victory for Barack Obama, but its historic worth and impact on the US president's legacy may not be known for a decade or more.

In July 2007, a dark-haired, fresh-faced US Senator was asked if he would meet the leaders of Iran, Syria, Venezuela, Cuba and North Korea without precondition in the first year of his presidency.

"I would", he shot back without hesitation. The idea that you punish countries by not talking to them "is ridiculous."

After more than 2,600 days in the Oval Office, Obama has publicly greeted only two of those leaders -- those of Venezuela and Cuba -- and then only briefly.

Tempered by experience and the weight of office, Obama's eagerly outstretched arm has evolved into a tougher doctrine of engagement coupled with "strategic patience."

The apogee of that doctrine is a political deal with Iran that appears near completion.

The agreement would limit Tehran's nuclear program for 10 to 15 years in return for sanctions relief.

"It would be a historical achievement," said Suzanne Maloney, an Iran expert at the Brookings Institution.

"It would be a vindication of the approach of engaging with regimes that we have deemed as rogue states or simply as adversaries."

But in essence, the deal is a postponement.

It's a bet that in just over a decade Iran's government will be less hardline and perhaps more willing to dismantle, not just limit, controversial aspects of its nuclear program.

It's a bet on patience.

"It's a large gamble," said Gary Samore, a White House non-proliferation advisor during Obama's first term.

"Iran is not making a strategic decision to abandon its interest in acquiring nuclear weapons, they are deciding for tactical reasons to accept temporary constraints on the program in exchange for sanctions relief."

- Serious conflagration -

Samore says it may take a decade or more before we know if the gamble has paid off.

"Whether it ends up being successful or not no one can say. We are not going to know during President Obama's presidency."

In the 1990s a similar deal with North Korea fell to pieces and Kim Jong il acquired a bomb.

Repeating that would be a disaster for Obama's legacy and could precipitate the serious military conflagration with Iran that he now seeks to avoid.

Alternatively, a deal that is honored could help repair ties severed after the 1979 Islamic Revolution and the and subsequent 444 day detainment of 52 US hostages in Tehran.

That in turn could redraw strategic alliances in the Middle East. Iran has only become more powerful since the collapse of Saddam Hussein's regime in Iraq.

"There is going to be a rise of Iran, no matter which way you cut it," said Hillary Mann Leverett, a former advisor on the National Security Council.

For Mann Leverett, president Richard Nixon's visit 1972 visit to China might be a more apt historical parallel for Obama than a failed deal with North Korea.

"The question is: can the US work with Iran for the constructive peaceful rise of Iran in a region that works more together, economically, politically and in security terms for the next generation. That's exactly what we did in Asia."

The deal will first have to survive opposition from Obama's allies in Israel and the Arab Gulf states.

In the short term, measures to reassure those allies is likely to put the deal under strain.

"The immediate consequence of a nuclear deal will be to intensify tension between the US and Iran," said Samore.

"I would expect that you would see intensified US efforts to challenge Iranian influence in Syria, in Iraq, Lebanon, Yemen and so-forth and certainly to strengthen security assurances to the Arab Gulf states."

Already Obama's White House is stressing that a "grand bargain" ending 30 years of animosity will take more than a nuclear agreement.

At a minimum, Tehran's leaders will have to stop threatening Israel and halt support for terror groups.

Getting to a point where that may be under discussion may prove the lasting achievement of Obama's open foreign policy.