US President Donald Trump has announced he's pulling out of the Iran nuclear deal.
Mr Trump spoke out against the arrangement as "a horrible, one-sided deal" based on a lie.
He said that if he allowed the deal to stand, there would soon be a nuclear arms race.
He called Iran a "regime of great terror" and further added: "no action taken by the regime has been more dangerous than its pursuit of nuclear weapons and the means of delivering them".
The move goes against advice from his European allies and some military advisers.
Mr Trump signed a presidential memorandum withdrawing from the 2015 agreement and said he is planning to reinstall sanctions on the Iranian regime.
He said in an address to the nation that he will be reinstating the highest level of sanctions and warned other countries not to help the Iranian government.
Mr Trump said America "will not be held hostage to nuclear blackmail" and will not allow "a regime that chants Death to America" to get access to nuclear weapons.
The president said he made the decision after consulting with US allies.
Mr Trump said "great things" can happen for the Iranian people following his announcement.
He predicted that Iranians would someday "want to make a new and lasting deal" and that "when they do, I am ready, willing and able".
He added that a new deal could lead to the "peace and stability we all want in the Middle East".
Mr Trump was speaking from the White House when he denounced the previous Iran deal as "defective at its core".
Despite lobbying from European allies, Mr Trump moved forward with his campaign promise to pull out of the Obama-era agreement.
The Iranians have been sharply critical of the Republican president's plan to withdraw.
Senator Dick Durbin of Illinois, the number-two Democrat in the Senate, branded the US withdrawal from the Iran nuclear deal "a mistake of historic proportions".
He said that breaking the Iran deal increases the danger that Iran will restart its nuclear weapons programme, which threatens Israel and "destabilises the entire Middle East".
Mr Durbin said Mr Trump's action "isolates the United States from the world at a time when we need our allies to come together to address nuclear threats elsewhere, particularly in Korea".
Mike Pompeo, US Secretary of State said: "As we exit the Iran deal, we will be working with our allies to find a real, comprehensive, and lasting solution to the Iranian threat.
"We have a shared interest with our allies in Europe and around the world to prevent Iran from ever developing a nuclear weapon.
"But our effort is broader than just the nuclear threat and we will be working together with partners to eliminate the threat of Iran’s ballistic missile program; to stop its terrorist activities worldwide, and to block its menacing activity across the Middle East and beyond.
"As we build this global effort, sanctions will go into full effect and will remind the Iranian regime of the diplomatic and economic isolation that results from its reckless and malign activity."
France, Britain and Germany regret Donald Trump's decision to withdraw from the Iran nuclear deal, French President Emmanuel Macron has said.