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.
Opposition politicians have called for a parliamentary inquiry into the handling of a radiation leak at a nuclear test reactor.
Public confidence has been damaged by the incident at the Vulcan Naval Reactor Test Establishment in Dounreay, Caithness, according to shadow defence secretary Vernon Coaker and shadow Scottish secretary Margaret Curran.
UK Defence Secretary Philip Hammond revealed details of a small internal leak of radiation on Thursday as he announced that the nuclear submarine HMS Vanguard is to have its reactor refuelled at a cost of £120 million.
Mr Coaker and Ms Curran said there were "significant questions" about the handling of the incident, particularly the two-year delay in making it public.
Russia may suspend nuclear arms inspections in a treaty with the US in reaction to Western sanctions over Ukraine, Russian news agencies quoted an unnamed defence ministry source as saying.
"The ungrounded threats to Russia from the US and NATO over its Ukrainian policy are regarded by us as an unfriendly gesture and allow us to announce force majeure," the unnamed source was quoted as saying by the state-owned RIA Novosti news agency.
"We are ready to take this step as a response to the Pentagon's statements about suspension of engagements between the armies of Russia and the US," the source added.
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.