Tehran and Washington have discovered a surprising common bond: to pretend that they might be heading toward serious negotiations to curb Iran's nuclear capacity. What's more, they are pretending for the same reason: to ward off an Israeli attack on Iran.
Their moves are barely noticeable—vague diplomatic pronouncements, op-eds, lots of behind-the-scenes orchestration by Russia. They don't want much attention—just enough to persuade Israel to wait on military action, to buy time. The American line is that the economic sanctions are working and weakening Tehran's will. Iran's line is we're willing to compromise, but we're not going to be pushovers.
Of course, there is no actual collusion between Iran and the United States; they don't trust each other. But both have reached the conclusion that war is worse than continued uncertainty—at least for the time being, as far as the United States is concerned.