According to a senior U.S. State Department official, Kerry told Zarif there could be no more delay. President Barack Obama's administration would call for even tighter sanctions on Iran unless a deal was reached now. Congress members were demanding new sanctions and the White House would join them.
Kerry made the case that "there would be no way to hold back new sanctions to give room for (a) new round and we would lead the charge for more sanctions if we did not come to agreement," the State Department official said.
By Saturday evening, the final language was personally approved by Obama in Washington. In a sign of how big a risk the Obama administration was taking, the main U.S. ally in the Middle East, Israel, decried what it called an "historic mistake", easing sanctions without dismantling Iran's nuclear programme.
But Obama said the deal put limits down on Iran's nuclear programme that would make it harder for Tehran to build a weapon and easier for the world to find out if it tried.
"Simply put, they cut off Iran's most likely paths to a bomb," Obama said in a late-night appearance at the White House after the deal was reached.
Obama was not the only one taking a risk. Iran's new president, the relative moderate Hassan Rouhani, was elected in June and inaugurated in August promising to ease the crippling sanctions. But Iran has invested billions of dollars in a nuclear programme, which its clerical and military establishment believes is a cornerstone of national pride.
Before Zarif was sent to Geneva, he and Rouhani had a meeting with Iran's supreme leader, Ayatollah Ali Khamenei, whose approval was absolutely required for any deal.
"The leader's main concern is his core supporters, who truly believe that there should be no deal with America, and are closely watching the developments to find a weak point or a failure to blame on the negotiators for betraying the leadership," said a former Iranian official, a relative of Khamenei.
The deal was in part the result of months of secret talks held with Iran in such out-of-the-way places as Oman, with U.S. officials using military planes, side entrances and service elevators to avoid giving the game away.
The talks, the most important contacts in more than three decades during which Iran branded the United States the "Great Satan" and the United States described Iran a part of an "axis of evil" that also