When campaigning for the presidency in 2016, Donald Trump consistently told his supporters that he would scrap the Iran nuclear deal that Barack Obama had worked so hard to achieve.
The businessman told fans he would "dismantle the disastrous deal with Iran" when he became president, but many world leaders hoped it was campaign rhetoric and that he would not follow through.
Diplomats are now trying to convince the president to reconsider his stance on the agreement in order to save the deal.
What exactly is the Iran nuclear deal?
In 2015 the UN Security Council's five permanent members - the US, UK, Russia, China and France, as well as Germany (known collectively as P5+1) - made an agreement to lift sanctions on Iran that were crippling its economy.
Prior to the deal being struck, Iran was progressing with a nuclear energy plan that world leaders feared could morph into a programme designed to manufacture nuclear weapons.
In return for the promise of reconnecting Iran to global markets, the country agreed to significantly cut its nuclear development.
It unplugged two-thirds of its centrifuges and shipped out nearly all of its enriched uranium, meaning all key components for nuclear weapons were removed.
Why did Iran agree to the deal?
The devastating economic sanctions imposed by the United Nations, United States and the European Union are estimated to have cost it tens of billions of pounds a year in lost oil export revenues.
Billions more in overseas assets had also been frozen, meaning the country was struggling to support itself financially.
Iran has huge oil reserves and in order to profit from export, the country needed to be reconnected to the global market. It agreed to the deal and all nuclear-related sanctions were lifted in January 2016.
Why does Trump think the deal is the "worst" the US has "ever" signed?
The deal was the key foreign policy achievement of Barack Obama's presidency, but Mr Trump believes the deal has "disastrous flaws" that have allowed Iran to flout the terms.
He waived sanctions on Iran in January, as he is required to do every 120 days, but said "this is a last chance" and asked "European countries to join with the United States in fixing significant flaws in the deal".
Israeli prime minister Benjamin Netanyahu says Iran tried to develop an atomic bomb before the historic agreement in 2015 and claimed the country was secretly storing "material to use at a time of its choice to develop nuclear weapons".
He gave no explicit evidence that Iran has violated the landmark agreement, but said it showed the country had clearly lied in the past and could not be trusted.
The European Union’s foreign affairs chief said Netanyahu’s allegations do not appear to show Tehran is currently violating the 2015 international nuclear agreement.
Iran denied the claims but the US said the evidence was "compelling".
What happens if the US walks away?
It is unclear exactly what would happen if the US walked away from the deal, however Iran has said the US re-imposing sanctions effectively amounts to "killing the deal".
Boris Johnson appeared on Mr Trump's favourite TV programme at the weekend in a final bid to stop the president scrapping the deal.
The Foreign Secretary told the Fox & Friends morning news show that Iran had been “behaving badly” and flaws in the deal needed to be fixed - but warned Mr Trump against ditching it.
He explained that “of all the options we have for ensuring that Iran never gets a nuclear weapon, this pact offers the fewest disadvantages”.
According to the White House, before 2015 Iran had enough enriched uranium and centrifuges to make 10 bombs.
Aside from preventing Iranian nuclear weapons, EU countries are concerned the US re-imposing sanctions would kill European hopes of building business ties with Tehran.
Evidence suggests that even if the US walks away from the deal, the agreement will be safe as long as no other countries follow suit.
Iran president Hassan Rouhani said he would be willing to keep to his part of the deal, provided the European Union offers guarantees that Tehran would keep benefiting from the accord.
He said: "What we want for the deal is that it's preserved and guaranteed by the non-Americans" - a reference to other signatories of the 2015 agreement.
He added that in this case "then the US pullout will be OK".
The foreign ministers of France and Germany said later that they will stick to the nuclear agreement, regardless of Mr Trump's decision.
France's top diplomat, Jean-Yves Le Drian, said the three European countries that were part of the deal - France, Germany and the UK - are committed to maintaining it.
Mr Le Drian said: "We will continue it independently of the American decision."
His German counterpart, Heiko Maas, said Berlin also wants to stick by the deal, which "makes the world a safer place and without it the world would be less safe".