So why didn’t the Obama administration lay down an even stronger marker in response to this breakout — by threatening, say, to intercept ships at sea that it believed were carrying parts for the Iranian nuclear program?

The answer, explained the senior official in a telephone interview, is that the U.S. wants to preserve consensus among its allies for much harsher sanctions, even as it heads toward a face-to-face negotiating meeting with the Iranians on Oct. 1.The U.S. has privately communicated with the Iranians in recent days that it wants those talks to go forward, the senior official said…

If the negotiations fail and Iran makes a further breakout toward weapons capability, “we could always escalate,” says the senior administration official. It’s hard to see how this one will end short of military confrontation if the Iranians don’t start bargaining for real.