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.
A spokesperson for the White House has confirmed that the US will follow through on its commitment to reduce some of the sanctions against Iran as laid out in a recent nuclear deal.
The spokesperson added that the US and its partners will "continue our aggressive enforcement" of the sanctions that remain in place.
A State Department source said that the US was "clear-eyed" about the difficulties that remain in addressing international concerns about Iran's nuclear programme.
Iran has said it will suspend its uranium enrichment in a few hours once talks with the UN nuclear watchdog have ended, the country's Fars news agency has reported, quoting an official.
– Behrouz Kamalvandi, spokesman for Iran's Atomic Energy Organisation
The International Atomic Energy Agency and our experts are in technical talks ... after that ends, I believe the suspension will start around noon (local time).
A landmark deal curbing Tehran's nuclear programme is being implemented by world powers and Iran.
Iran will begin to freeze part of its nuclear programme in return for the sanctions relief from January 20.
The deal, agreed between Iran and world powers after months of negotiation led by the EU's foreign policy chief Catherine Ashton, is hinged on Iran curbing its enrichment of uranium.
Officials have another six months from January 20 to agree a final settlement about the activity the West suspects is aimed at obtaining nuclear weapons capability.
William Hague has hailed implementation of an interim deal to freeze Iran's nuclear programme.The Foreign Secretary said the agreement coming into force was an "important step" in settling the differences between the Middle East and international powers.
The entry into force of this agreement on January 20 is an important step towards peacefully resolving the Iranian nuclear issue, on which comprehensive negotiations will now start.
The United States said that although sanction relief will begin immediately the full benefits will not be released until the deal reaches its end.
– US State Department
The United States and the rest of our P5+1 partners will also take steps, in response to Iran fulfilling its commitments, to begin providing some limited and targeted relief.
The $4.2 billion in restricted Iranian assets that Iran will gain access to as part of the agreement will be released in regular installments throughout the six months.
The final installment will not be available to Iran until the very last day.
Iran will start to receive sanction relief from the first day of the interim nuclear deal, 20th January a US official has confirmed.
Once the IAEA confirms that Iran has implemented the deal on 20th January, the US will suspend some petrochemical and precious metal sanctions.
An interim nuclear deal between Iran and six world powers and Iran will start on 20th January, the EU's foreign policy chief Catherine Ashton said.
Ashton represents the six nations - the United States, Russia, China, France, Britain and Germany - in diplomatic contacts with Iran related to the nuclear standoff.
She said the sides would now ask the United Nations' nuclear watchdog to verify the deal's implementation.
"We will ask the IAEA to undertake the necessary nuclear-related monitoring and verification activities," she said.
Under the agreement, Iran has promised to curb its most sensitive nuclear activities in return for some relief from Western economic sanctions.
Mexico has issued a public alert after a truck carrying potentially "extremely dangerous" radioactive material was stolen on Monday, according to Mexican authorities.
The vehicle was carrying cobalt-60 from a hospital in the northern city of Tijuana to a waste storage site.
A statement from the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), said:
– iaea statement
At the time the truck was stolen, the source was properly shielded. However, the source could be extremely dangerous to a person if removed from the shielding, or if it was damaged.
The Mexican authorities are currently conducting a search for the source and have issued a press release to alert the public.
President Barack Obama has shrugged off criticism of the deal to limit Iran's nuclear development - saying it's good for American security.
The deal has been criticised by America's Middle East allies Israel and Saudi Arabia.
Foreign Secretary William Hague said the nuclear deal with Iran does not mean the UK embassy in Tehran will reopen.
"We have to be clear with Iran that an embassy, if we reopen it, would be able to operate properly with all the normal functions. We will take this in a step by step way," he said.