WASHINGTON — It was Secretary of State Hillary Rodham Clinton who, nearly a year ago, fired the Obama administration’s first warning shots about imposing “crippling sanctions” against Iran.

Then came President Obama’s declaration that if Iran did not respond to his overtures for a negotiated end to its production of nuclear fuel by the end of 2009, he would quickly add a “pressure track” to his diplomacy. At the beginning of this year, American and European officials predicted a sanctions resolution at the United Nations Security Council — the fourth in four years — by February to pave the way for much stronger crackdowns by individual countries.

Now, all those projections look optimistic. The French are predicting...