In the end, the deal was written entirely by the U.S. side. The Russians agreed to it in an impromptu poolside conversation between Kerry, Lavrov and their deputies, who dragged over chairs to join them. Kerry made final edits to the draft on an iPad in his hotel room.

The version of events offered Sunday by the senior State Department official, who spoke on condition of anonymity to discuss the closed-door conversations, was both riveting and self-serving. The Russians offered no recounting of their own, except to emphasize that the deal was their idea, discussed in general terms over the past year but not seriously addressed until they proposed it last week…

While Kerry and Lavrov dined on fish and salad, the U.S. team came out of an initial session with their Russian counterparts “feeling both sides were talking past each other and we needed to figure out a different way to break through. This wasn’t going to work on the current path.”

The two sides were “miles away” from each other on their assessments of the size of the weapons stockpiles. The team called in intelligence analysts from both sides, who sat down to thrash out their differences.