Does the proposed deal with Iran actually prevent the Mullahs from ever developing a nuclear weapon? Or does it merely delay them for a period of years? That is the key question that has not yet been clearly answered.

In his statement on the deal, President Obama seemed to suggest that Iran will never be allowed to develop a nuclear weapon. He said that this “long-term deal with Iran… will prevent it from obtaining a nuclear weapon.” He then repeated this assurance: “because of this deal, the international community will be able to verify that the Islamic Republic of Iran will not be able to develop a nuclear weapon.” These seemingly categorical statements were intended to assure the world that President Obama would keep his earlier promise that Iran will never be allowed to develop nuclear weapons.

But is that what the deal itself does? Or, as stated by its critics, does it actually assure that Iran will be allowed to develop a nuclear arsenal after a short delay of several years? That is the key question that the Obama administration has refused to answer directly. It must do so before Congress can be asked to buy a pig in a poke for the American people.

There is an enormous difference between a deal that merely delays Iran’s development of a nuclear arsenal for a period of years and a deal that prevents Iran from ever developing a nuclear arsenal. Prime Minister Netanyahu of Israel and many other critics of this deal describe it as merely a delay, while the Obama administration seems to be suggesting by its rhetoric that the deal will prevent Iran from ever obtaining a nuclear weapon.

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The devil is not so much in the details as in the broad outlines of this deal and its understanding by the parties. Does it or does it not allow Iran to develop nuclear weapons after a relatively short moratorium? Iran certainly seems to believe that it does, Israel certainly believes that it does, and many in Congress-both Republicans and Democrats– seem to believe that it does. But the President seems to be telling the American public and the world that these critics are wrong: that Iran will never be allowed to develop a nuclear weapon under this deal.

Yet, just a few months ago, he seemed more cautious and candid in discussing his “fear” that “in year 13, 14, 15, they have advanced centrifuges that enrich uranium fairly rapidly, and at that point the breakout times would have shrunk almost down to zero.” He also said that we have assurances of a yearlong breakout time “for at least well over a decade,” implying that after that indeterminate time frame, the assurances will no longer be in place.

Obama’s statement, despite its confusing and ambiguous context, has raised deep concerns among critics of the deal. Moreover, the text of the deal includes time frames of 8 years, 10 years and 15 years, which also generates confusion at a time when clarity is essential.

So which is it? Congress has a right to know, and so do the American people. Is it a postponement for an uncertain number of years – 8, 10, 13, 14, 15 – of Iran’s ability to develop a nuclear weapon? Or is it an assurance that “Iran will not be able to develop a nuclear weapon?”

The Obama administration insists that this is not a “treaty,” but rather a “deal.” A deal is a contract, and for a contract to be valid there must be a “meeting of the minds.” But has there been a meeting of the minds over the central issue of whether this deal allows Iran to develop a nuclear weapon after a moratorium whose precise time frame is unclear? And if there has been a meeting of the minds over this issue, what exactly is it?

Certainly the words of the Iranians are not the same as the words of President Obama. Whose words accurately represent the meaning of the contract we are being asked to sign?

The time has now come to be crystal clear about the meaning of this deal. If it is intended to prevent Iran from ever developing nuclear weapons, the President must say so in the clearest of terms and he must get the Iranians to express agreement with that interpretation. Ambiguity may be a virtue at the beginning of a negotiation, but it is a vice in interpreting and implementing a deal with such high stakes.

Recall that President Bill Clinton made similar assurances with regard to North Korea back in 1994 – as the chart below shows. But within a few short years of signing a deal that he assured us would require the dismantling of North Korea’s nuclear program, that country tested its first nuclear weapon. It now has a nuclear arsenal. How can we be sure that Iran will not act in a similar fashion?

The deal with Iran has been aptly characterized as a “leap of faith,” “a bet” and a “roll of the dice” by David Sanger in a news analysis for the New York Times. The gamble is that by the time the most restrictive provisions of the deal expire, Iran will be a different country with more reasonable leaders. But can the world and especially the nations most at risk from an Iranian nuclear arsenal, depend on faith, bets and dice, when they know that the last time the nuclear dice were rolled, they came up snake-eyes for America and its allies when North Korea ended up with nuclear weapons.

The burden of persuasion is now on the Obama administration to demonstrate that President Obama was accurately describing the deal when he said that it will “prevent” Iran from “obtaining a nuclear weapon.” It is a heavy burden that will be – and should be – difficult to satisfy.

Alan Dershowitz is Professor Emeritus at the Harvard Law School and author of Taking the Stand: My Life in the Law.