Transcript for Trump withdraws US from Iran nuclear deal

And we begin with president trump's major announcement today, being watched all over the world. Pulling the U.S. Out of the Iran nuclear deal and imposing tough new sanctions on Iran instead. The president in the speech seen here in the U.S. And carried live in Iran, calling the deal disastrous. America's Al Littles had urged the president not to do this. They are reacting tonight, and so is Iran already with a new threat. ABC's chief white house correspondent Jonathan Karl leading us off. Reporter: Defying our allies and keeping a campaign promise, president trump is taking the United States out of the Iran nuclear deal. The fact is, this was a horrible, one-sided deal that should have never, ever been made. It didn't bring calm. It didn't bring peace. And it never will. Reporter: This is no half measure. The president declared he is reimposing sanctions waived as part of the deal, all of them. America will not be held hostage to nuclear blackmail. Reporter: Under the Iranian nuclear deal, Iran agreed shut down its production of weapons-grade uranium and allowed regular inspections by the international atomic energy agency, which says Iran has stuck by its side of the deal. But the president says it falls short, that it doesn't prevent Iran from supporting terrorism or building up its missile program. At the heart of the Iran deal was a giant fiction that a murderous regime desired only a peaceful nuclear energy program. Reporter: Anguished by today's decision, former president Obama. The Iran deal, his chief foreign policy accomplishment. Today in a statement, he warns, "The United States could eventually be left with a losing choice between a nuclear armed Iran or another war in the Middle East." And tonight, from Iran's president, an ominous threat -- hassan rouhani announcing he's ordered Iran's scientists to prepare to start enriching uranium again if necessary. When I recently asked the president about that prospect, he answered with a threat of his own. But the Iranians say they'll restart their nuclear program if the deal -- We'll find out. You'll find out about that. Reporter: Are you concerned about that? Won't be so easy for them to restart. They're not going to be restarting anything. They restart it, they're going to have big problems. Bigger than they ever had before. Reporter: Today, the president cast his actions on Iran as a message to north Korea. The United States no longer makes empty threats. When I make promises, I keep them. Reporter: The president announced secretary of state Mike Pompeo is in North Korea again tonight, laying the groundwork for the much anticipated nuclear summit with Kim Jong-un. Jon Karl live with us from the white house tonight. Iran agreed to the robust nuclear inspections. International inspectors found Iran was in compliance with the deal. And the president still pulling out today. Now North Korea on the agenda. How is the president going to convince North Korea to agree to even the same level of inspections Iran was already allowing? Reporter: David, that may be the single most important unanswered question, as the president goes into his meeting with Kim Jong-un. After all, North Korea already has nuclear weapons, and it is the most isolated secretive nation on Earth. Jon Karl leading us off tonight. Jon, thank you. American allies urged the president not to do this, to stay in the deal. Our chief foreign correspondent Terry Moran live in London tonight. And our allies already responding tonight? Reporter: That's right, David. They are. Dismayed and bewildered right now. You saw the European leaders getting the red carpet treatment at the white house, chancellor Merkel of Germany meeting with president trump, trying to change his mind, because they and every other country that signed this deal think this deal is working. They say it's put Iran's nuclear program in hand cups and it's been inspected and it should continue. There's also a sense of loss, almost, that as they go forward, America under president trump isn't a reliable partner in these big international agreements. And their big fear, that what he really means by a solution to the problem is regime change in Tehran, and that could make the whole Middle East a much more dangerous place. And Terry, one more question while we have you. Viewers will remember our trip to Iran. We witnessed the sanctions before this nuclear deal was even negotiated. It was just days before the negotiations began, in fact. The crowds waiting for medicine, cancer treatment. We heard president trump today telling the Iranian people, the people of America stand with you, but how is this decision being received on the streets of Tehran tonight? Reporter: Anger and defiance, David. They do not believe that claim. Social media in Iran just lit up with a lot of anti-american tweets, that's no surprise, saying America's arrogant and can't be trusted. A lot of tweets on those issues you mentioned. People concerned about the life-saving medications they or their parents get. People worried about hardliners in Iran cracking down harsher than they have before, and that fear of war. David? Terry Moran with us tonight on this, as well. Terry, thank you.

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