Only hours after the conclusion of an agreement with Iran to lift oil and financial sanctions in return for curbs on Iran's nuclear capabilities, US President Barack Obama is a man who evinces no second thoughts whatsoever about the deal he struck.

In a 45-minute interview in the Cabinet room, the President kept stressing one argument: Don't judge me on whether this deal transforms Iran, ends Iran's aggressive behaviour towards some of its Arab neighbours or leads to detente between Shi'ites and Sunnis. Judge me on one thing: Does this deal prevent Iran from breaking out with a nuclear weapon for the next 10 years and is that a better outcome for America, Israel and our Arab allies than any other alternative on the table?

The President made clear that he did not agree with my assessment in a previous column that we had not used all the leverage in our arsenal, or alliances, to prevent Iran from becoming a threshold nuclear power, by acquiring a complete independent enrichment infrastructure that has the potential to undermine the global nuclear non-proliferation regime.

Personally, I want more time to study the deal, hear from the non-partisan experts, listen to what the Iranian leaders tell their own people and hear what credible alternative strategies the critics have to offer.

But the President certainly argued his case with a conviction and internal logic with which his critics and Congress will have to seriously contend.

"We are not measuring this deal by whether it is changing the regime inside of Iran," said the President. "We're not measuring this deal by whether we are solving every problem that can be traced back to Iran, whether we are eliminating all their nefarious activities around the globe. We are measuring this deal - and that was the original premise of this conversation, including by Prime Minister Netanyahu - (on the basis that) Iran could not get a nuclear weapon. That was always the discussion. And what I'm going to be able to say, and I think we will be able to prove, is that this, by a wide margin, is the most definitive path by which Iran will not get a nuclear weapon, and we will be able to achieve that with the full cooperation of the world community and without having to engage in another war in the Middle East."

To sell this deal to a sceptical Congress, Mr Obama clearly has to keep his argument tight. But I suspect his legacy on this issue will ultimately be determined by whether the deal does, in the long run, help transform Iran, defuse the US-Iran cold war and curtail the spread of nuclear weapons in the Middle East - not foster their proliferation.

That, though, will be a long time in determining. For the near term, the deal's merit will be judged on whether Iran implements the rollback of its nuclear enrichment capabilities to which it has agreed and whether the deeply intrusive international inspection system it has accepted can detect - and thereby deter - any cheating.

Here are some highlights from the interview:

Asked about whether we failed to use all of our leverage, including a credible threat of force, the President said: "I think that criticism is misguided. Let's see exactly what we obtained. We have cut off every pathway for Iran to develop a nuclear weapon. The reason we were able to unify the world community around the most effective sanctions regime we've ever set up, a sanction regime that crippled the Iranian economy and ultimately brought them to the table, was that the world agreed with us that it would be a great danger to the region, to our allies, to the world, if Iran possessed a nuclear weapon. We did not have that kind of global consensus around the notion that Iran can't enjoy any nuclear power whatsoever. And as a member of the Non-Proliferation Treaty, the NPT, their argument was, 'We're entitled to have a peaceful nuclear programme.'

"And what we were able to do," the President continued, "is to say to them, 'Given your past behaviour, given our strong suspicion and evidence that you made attempts to weaponise your nuclear programme, given the destabilising activities that you've engaged in in the region and support for terrorism, it's not enough for us to trust when you say that you are only creating a peaceful nuclear programme. You have to prove it to us.'

"And so this whole system that we built is not based on trust, it's based on a verifiable mechanism, whereby every pathway that they have is shut off."

The President argued that his approach grew out of the same strategic logic that presidents Richard Nixon and Ronald Reagan used to approach the Soviet Union and China.

"You know, I have a lot of differences with Ronald Reagan, but where I completely admire him was his recognition that if you were able to verify an agreement that (was negotiated) with the evil empire that was hell-bent on our destruction and was a far greater existential threat to us than Iran will ever be", then it would be worth doing, Mr Obama said. "I had a lot of disagreements with Richard Nixon, but he understood there was the prospect, the possibility, that China could take a different path. You test these things, and as long as we are preserving our security capacity - as long as we are not giving away our ability to respond forcefully, militarily, where necessary to protect our friends and our allies - that is a risk we have to take. It is a practical, common-sense position. It's not naive; it's a recognition that if we can in fact resolve some of these differences, without resorting to force, that will be a lot better for us and the people of that region."

Asked if Russian President Vladimir Putin was a help or a hindrance in concluding this deal, Mr Obama said: "Russia was a help on this. I'll be honest with you. I was not sure, given the strong differences we are having with Russia right now around Ukraine, whether this would sustain itself. Putin and the Russian government compartmentalised on this in a way that surprised me, and we would have not achieved this agreement had it not been for Russia's willingness to stick with us and the other P5-Plus members in insisting on a strong deal. I was encouraged by the fact that Mr Putin called me a couple of weeks ago and initiated the call to talk about Syria. I think they get a sense that the Assad regime is losing a grip over greater and greater swathes of territory inside of Syria (to Sunni jihadi militias) and that the prospects for a (Sunni jihadi) takeover or rout of the Syrian regime is not imminent but becomes a greater and greater threat by the day. That offers us an opportunity to have a serious conversation with them."

My biggest concern, and that of many serious critics who would actually like to see a deal work, is that Iran is just not afraid of a serious US military retaliation if it cheats. I asked the President: Why should the Iranians be afraid of us?

"Because we could knock out their military... if we chose to," said the President, "and I think they have seen my willingness to take military action where I thought it was important for US interests. Now, I actually believe that they are interested in trying to operate on parallel levels, to be able to obtain the benefits of international legitimacy, commerce, reduction of sanctions while still operating through proxies in destructive ways around the region. That's been their pattern, and I think it is very important to us to make sure that we are surfacing what they do through their proxies and calling them to account. That is part of the conversation we have to have with the Gulf countries."

The President spoke to Prime Minister Benjamin Netanyahu of Israel by phone just before the start of the interview. Mr Obama did not try to sugar-coat their differences, but hinted that his administration has in the works some significant strategic upgrades for both Israel and America's Gulf allies.

"I don't think it's appropriate for me to discuss specific details about security agreements or work that we may be doing," said the President. "What I can tell you is that that process is in train. Now, with respect to the Israelis, I think it's fair to say that under my administration, we've done more to facilitate Israeli capabilities. And I've also said that I'm prepared to go further than any other administration's gone before in terms of providing them additional security assurances from the US. The thing I want to emphasise is that people's concerns here are legitimate. Hizbollah has tens of thousands of missiles that are pointed towards Israel. They are becoming more sophisticated. The interdiction of those weapon flows has not been as successful as it needs to be. There are legitimate concerns on the part of the Gulf countries about Iran trying to stir up and prompt destabilising events inside their countries. So they're not just being paranoid. Iran is acting in an unconstructive way, in a dangerous way in these circumstances. What I've simply said is that we have to keep our eye on the ball here, which is that Iran with a nuclear weapon will do more damage, and we will be in a much worse position to prevent it."

It strikes me that the one party that we have heard the least from, but in the end could count the most, is the Iranian people and how they react to this opening of Iran to the world as a result of this deal. What would Mr Obama say to them?

"What I'd say to them is this offers a historic opportunity," the President said. "Their economy has been cratering as a consequence of the sanctions. They have the ability now to take some decisive steps to move towards a more constructive relationship with the world community... They need to seize that opportunity, their leaders need to seize that opportunity. And the truth of the matter is that Iran will be and should be a regional power. They are a big country and a sophisticated country in the region. They don't need to invite the hostility and the opposition of their neighbours by their behaviour. It's not necessary for them to be great to denigrate Israel or threaten Israel or engage in Holocaust denial or anti-Semitic activity. Now that's what I would say to the Iranian people. Whether the Iranian people have sufficient influence to fundamentally shift how their leaders think about these issues, time will tell."

NEW YORK TIMES