If the question of the Iranian nuclear program is gaining urgency — as it seems to be doing in the wake of the exercises by the Israel Defense Forces off the Greek Mediterranean reported last week by the New York Times — here is the reason: It is a growing sense that Iran doesn’t need to develop an atomic bomb even roughly like the one we dropped over Hiroshima. What worries Israeli leaders is that, even if the technology to craft the kind of warhead that produces an explosion is still a few years away, the mullahs are close to testing a “nuclear yield.” One way to think of that is to comprehend what happened on October 9, 2006, when North Korea demonstrated something that was not a weapon in the traditional sense but a “nuclear device.”

The ambiguity over what is and isn’t a weapon — North Korea’s, our spies reckon, produced a pfffffft that was less than India’s weapon but still dangerous — has befuddled our national intelligence bureaucrats. In December, they released a declassified estimate that claimed Iran had halted its weapons program in 2003, even though it acknowledged in a footnote to that sentence that Iran was enriching uranium out in the open at Natanz. In February, the director of national intelligence told the Senate that he regretted the wording of his analysts, insisting that the “long pole of the tent” for bomb making — enrichment — continued.

The next day, speaking before the House Permanent Select Committee on Intelligence, the director, Admiral McConnell, said estimates within the intelligence community vary as to when Iran will test an explosive, to between 2010 and as late as 2015. But, the admiral warned, Iran could theoretically master the technology for a nuclear yield as early as 2009. “Theoretically, they could do it, given their current effort, by 2009,” Admiral McConnell told Rep. Heather Wilson. “We don’t think they’re moving quite that fast, but we don’t have perfect insight and understanding.”

It was earlier in his colloquy with Ms. Wilson that the director let slip Israel’s worst nightmare. “If the intent was to do nothing but have a nuclear yield, just a yield, something that you could haul around on a truck or bury in the ground, they could do that in six months to 12 months,” he said. No wonder, as our Eli Lake reported earlier this month, the United States Coast Guard has already started boarding any commercial vessels that have been to Iranian ports within five calls before they enter our docks and harbors.

No wonder also that three of Senator Obama’s top foreign policy advisers, Richard Clarke, Tony Lake, and Susan Rice, signed a Washington Institute for Near East Policy position paper calling on the current president and the next one to establish a senior channel with Israel to discuss not simply the deterrence of but the prevention of an Iranian A-bomb. No wonder the Israelis are practicing the exact operations they would need to perform to take out the facilities that would produce the fissile material needed for a demonstration of what would amount to Iranian hegemony.

The latest estimates in Israel are now measuring the timeline for an Iranian nuclear yield in months. According to a story on page one of Sunday’s Jerusalem Post, the Israeli calculation is that the Iranians will be to able master the enrichment technology by as early as this year and likely in 2009. No intelligence estimate is certain. But this is a question the Jewish state cannot get wrong. It is also something Israeli war planners have been thinking about since the mid-1980s, when America began selling Israel the F15s and F16s necessary for the kind of mission Israel has just practiced over the Mediterranean.

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One of the things to bear in mind is that the logic of mutual assured destruction isn’t adequate here. Merely the testing of a nuclear device by Iran would alter the daily calculus, enabling the regime’s president to add a great deal more credibility to his promises to destroy the Jewish state. The threat alone could reverse Israel’s post-Oslo economic growth as international investors reconsider placing a bet on a country that is the target of madmen. It could also trigger a kind of reverse biblical exodus from the Promised Land. Finally it would give Iran’s two terrorist proxies on Israel’s flanks, Hamas and Hezbollah, a nuclear umbrella. Even the announcement by Iran of a nuclear yield would, proliferation experts agree, trigger a nuclear arms race in the center of the world’s energy supply. The consequences to America’s own interests would be enormous as allies such as Egypt, Turkey, and Saudi Arabia start nuclear weapons programs. No doubt this is all why pilots of the IDF wheeled into the skies over the Mediterranean to test their abilities to strike at Arak, Bushehr, and Natanz.