When you consider the diplomatic mountain the US and Iran have already scaled, you get a daunting sense of the challenge ahead.
The deal between Iran and world powers looks like a breakthrough, but with six months of wrangling over details ahead, it could still fail.
The nuclear agreement between Western powers and Iran was broadly welcomed by politicians, former world leaders and Middle East experts.
President Barack Obama has told Israeli Prime Minister Benjamin Netanyahu that he wants to begin US-Israeli consultations immediately on an effort to reach a comprehensive solution to Iran's nuclear programme, according to the White House.
Mr Obama reassured the Israeli leader that the United States will remain firm in its commitment to Israel, after Mr Netanyahu called the nuclear deal with Iran "an historic mistake".
"The president underscored that the United States will remain firm in our commitment to Israel which has good reason to be sceptical about Iran's intentions," said White House spokesman Josh Earnest.
President Barack Obama has spoken by phone with Israeli Prime Minister Benjamin Netanyahu to discuss a nuclear deal between Iran and six major powers.
The White House announced the call as Obama flew aboard Air Force One from Washington to Seattle.
Germany's Foreign Minister has welcomed a nuclear deal between Iran and six world powers in Geneva today, in what is being billed as one of the most significant developments with Iran in 30 years.
Guido Westerwelle told reporters:
The agreement of Geneva is a turning point in those difficult times and after very difficult talks. We are a step closer to our goal of preventing a nuclear armament of Iran.
Foreign Secretary William Hague has said the deal between Iran and world powers "vindicates the policy of pressure through sanctions and diplomacy through negotiations."
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The UK and US will ensure a deal on Iran's nuclear programme will be implemented and that it does not give the country any right to uranium enrichment, the Foreign Secretary said as he met with US Secretary of State John Kerry in London.
William Hague told reporters: "It is a very important opportunity for the future. And it vindicates in the future the policy of pressure through sanctions and diplomacy for negotiations in which the United State and United Kingdom have been strong partners for so long".
Prime Minister David Cameron has praised the work of William Hague and John Kerry following the deal between Iran and world powers, but adds its is "nowhere near the end".
There has been a "very different" and "sincere" approach from Iran since the government of president Hassan Rouhani took office, Foreign Secretary William Hague said today, after a landmark deal was reached over its nuclear programme.
Mr Hague told BBC Radio 4's World This Weekend :""This is a change in behaviour by Iran and we should respond constructively and openly to that.
This was "only a beginning", he stressed. "Can we reach such an agreement? It will be difficult but I think this agreement we reached in the early hours of this morning should give us faith in the power of diplomacy and pressure to resolve even some very intractable problems."
The international deal on Tehran's atomic ambitions leaves Iran "further away from getting a nuclear weapon", David Cameron has said.
The Prime Minister said the deal struck after lengthy negotiations in Geneva was an "important first step" and "demonstrates how persistent diplomacy and tough sanctions can together help us to advance our national interest".
He insisted sanctions would continue to be enforced "robustly" in order to secure a comprehensive final deal with Iran.
He said: "We now have an international agreement with Iran that moves it further away from getting a nuclear weapon.
"This is an important first step, which must now be fully implemented. We will continue to enforce sanctions robustly in order to secure a comprehensive and final settlement that fully addresses the real and substantive concerns of the international community.
"Today's deal with Iran demonstrates how persistent diplomacy and tough sanctions can together help us to advance our national interest."
The deal on Iran's nuclear programme leaves Tehran further from getting a nuclear weapon and "demonstrates how persistent diplomacy and tough sanctions can together help us to advance our national interest", Prime Minister David Cameron said.