His engagement – both in private and in public and according to aides, at a level of minute detail – is in contrast to a more aloof approach as Egypt came under military rule and Syria descended into civil war.

“It’s the top item on his foreign agenda for the rest of his term,” a source close to the White House’s thinking said of the Iran issue. “He doesn’t want to leave anything to chance.”

The stakes are enormous for Obama. If the talks break down and Iran dashes to build an atomic bomb before the West can stop it, he could go into the history books as the president whose naivete allowed the Islamic Republic to go nuclear.

The breakthrough with Iran is also worrying the many pro-Israel members of Congress, including heavyweights in his own Democratic Party like Senator Charles Schumer.

Last weekend’s Iran pact – a preliminary agreement on modest sanctions relief in exchange for temporary curbs on Iran’s nuclear activities – was no case of accidental diplomacy.