(CNN)It is the moment that all the grunting and yelling has been leading up to.

Few doubted the United States and Iran deep-down preferred diplomacy -- even a new deal -- to escalating tensions, and possibly military confrontation. Now the moment of truth is here, in a Vienna conference room, where the E3+2 of the nuclear deal (UK, France, Germany, Russia and China) and Iran will sit down and talk about what comes next.