Of all the arguments against last week’s deal to restrict Iran’s nuclear program, the strongest may be that it has an expiration date. President Obama confronted this criticism in his news conference about the agreement: “That’s a good one,” he said, without apparent condescension.

So why doesn’t he have a better answer for it?

The president’s basic answer is that, when the deal expires 15 years from now, his successor will be no worse off than he is.

Yes, Iran will be able to pursue a nuclear-weapons program. But it will be starting from where it is today. And it will still be subject to the Nuclear Non-Proliferation Treaty and to inspections.

That’s not a satisfying response. To understand why, it’s helpful to recall the original goal and history of these negotiations.

When they began, a top priority of the administration was to reduce Iran’s number of centrifuges that produce enriched uranium, as well as Iran’s stockpiles of the fuel. These are the two elements that determine the speed at which a country can potentially reprocess fuel to weapons-grade and build a bomb – “breakout” time, in nuclear-speak.

If Iran abides by the terms of the agreement – and its record of keeping promises is not exactly encouraging – its breakout time will go from about two months to a year.

Obama needs to address this looming threat – or his insistence that this agreement was the best course available to him will count for nothing. What will that matter, if the deal ultimately fails to prevent a nuclear-armed Iranian theocracy and an arms race in the volatile Middle East?