The hard work starts now, John Kerry said in London tonight. When you consider the diplomatic mountain the US and Iran have already scaled, you get a daunting sense of the challenge ahead.
So can the interim deal be turned into a lasting accord? Of course the details will be crucial; the tricky task of inspection and verification.
Then there will be the great dollops of re-assurance demanded by Israel and Arab nations who share profound concerns about Iran’s desire to dominate the region.Which leads to the most important and often elusive ingredient of all; trust.
Presidents Obama and Rouhani have staked their reputations on the next six months.But they are pushing against a great tide of history.Since the Islamic Revolution and the severing of ties between Iran and the US, the animosity between these two powers and their regional allies has shaped events in the region.
Most recently that means Syria where, put crudely, America and her Arab allies have been pitched against Bashar Al Assad and his Iranian friends.Now it could be that a thaw in US-Iranian relations shakes up that bloody stalemate. A renewed initiative, known as Geneva II, is due to be announced this week.
Warning: John Ray's report contains flash photography:
You have to be widely optimistic to believe that a practical peace deal is at hand. Indeed the re-arranging of regional alliances could make the situation more dangerous.But of all the generational changes underway in the Middle East these past three years, an end to the cold war between America and Iran could be just as important as any event of the Arab Spring.