Israel Matzav

For that, proponents of diplomacy over war with Iran can thank a man they have often ridiculed or reviled: Israeli Prime Minister Benjamin Netanyahu.

Mr. Netanyahu’s government is not a participant in the talks with Iran, of course; Iran won’t parley with a nation it aspires to “wipe off the map.” But the Israeli leader’s explicit setting of a “red line” for the Iranian nuclear program in a speech to the U.N. General Assembly in September appears to have accomplished what neither negotiations nor sanctions have yielded: concrete Iranian action to limit its enrichment.

A host of commentators both in the United States and Israel scoffed at what they called Mr. Netanyahu’s “cartoonish” picture of a bomb and the line he drew across it. The prime minister said Iran could not be allowed to accumulate enough 20 percent enriched uranium to produce a bomb with further processing, adding that at the rate its centrifuges were spinning, Tehran would cross that line by the middle of 2013.

Iran, too, dismissed what its U.N. ambassador called “an unfounded and imaginary graph.” But then a funny thing happened: The regime began diverting more of its stockpile to the manufacture of fuel plates for a research reactor. According to the most recent report of international inspectors, in February, it had converted 40 percent of its 20 percent uranium to fuel assemblies or the oxide form needed to produce them. As a result, Iran has remained distinctly below the Israeli red line, and it probably postponed the earliest moment when it could cross that line by several months.

Mr. Netanyahu’s red line is only a partial and temporary check on the Iranian threat. The ongoing installation of a new generation of faster centrifuges could soon make it obsolete by providing a new means for Iran to quickly produce bomb-grade uranium. But the lesson here is twofold: The credible threat of military action has to be part of any strategy for preventing an Iranian nuclear weapon, and clear red lines can help create the “time and space for diplomacy” that President Obama seeks. Mr. Obama, who last year stiffly resisted pressure from Mr. Netanyahu to spell out U.S. red lines, ought to reconsider.