Three months into intense international negotiations over Iran’s disputed nuclear development program, Tehran’s team has surprised almost everybody with its apparent eagerness for a deal. Iranian negotiators have met all of their commitments under November’s interim agreement, have proposed compromises on some key disagreements, and have taken part in three top-level meetings without the squabbles that were common over the last decade of fruitless haggling. Yet President Hassan Rouhani’s government is moving away from the United States and its allies on an issue that may be the most important of all. Put simply, the six world powers want Iran to curtail enrichment of uranium. They want Tehran to cut its 19,000 uranium-enriching centrifuges to a few thousand. Tehran, however, is insisting on vastly expanding capacity by adding thousands more centrifuges for what it says is strictly civilian energy purposes. […]