WASHINGTON — President Donald Trump announced Tuesday that he is withdrawing the United States from the Iran nuclear deal, reneging on a landmark pact and raising the question of whether Tehran might respond by resuming its frozen weapons program.

"It is clear to me that we cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of the current agreement," Trump said in a nationally televised statement delivered from the White House. "The United States will withdraw from the Iran nuclear deal."

Trump further said that he would impose the "highest level" of sanctions, which would affect Iran directly and other countries that do business with the regime. But he did not specify which sanctions he would put in place or whether there would be time for foreign countries and companies to disentangle ties to Iran.

In 2015, Iran agreed to halt its pursuit of nuclear weapons — and to allow international checks on its facilities — in exchange for moves by the U.S., several other countries and the United Nations to roll back sanctions that had crippled its economy. The accord was widely seen as the biggest foreign policy accomplishment of President Barack Obama's administration.

But Trump has long been a critic of the deal, maintaining that Obama gave up too much for too little. His decision signals a dramatic reversal of U.S. policy, and his rhetoric was hawkish.

If Iran doesn't get in line, he said, it will face "bigger problems than it has ever had before."

But the move also raises two important questions for the Trump administration and the international community: whether Iran will respond by resuming its quest for nuclear weapons and whether reneging on the Iran pact will affect North Korea's willingness to cut a denuclearization deal with the president.

"We are basically just going back on our word," said Jon Wolfstahl, who was the National Security Council’s senior director for arms control and nonproliferation under Obama.

It is difficult to say exactly how Trump’s decision is likely to play out. The U.S. sanctions regime is complex and only part of the international picture. Moreover, the reactions of each country involved in the original deal are difficult to predict precisely.

Many of the existing Iran sanctions are independent of the 2015 deal and remain in place, including a prohibition on the vast majority of direct commercial activity between the U.S. and Iran, according to David Mortlock, who was a sanctions expert on Obama’s National Security Council.

But the waiver that was due for renewal May 12 — the one that created the deadline prompting Trump’s action — is tied up in the Iran deal. Using the banking system, it effectively blocks implementation of U.S. sanctions on foreign countries that buy oil from Iran. Without that waiver, foreign countries can escape U.S. sanctions only if the president determines that they have reduced their purchases of Iranian oil.

In short, that means foreign countries and companies are now risking their ability to do business in the U.S. if they continue to buy Iranian oil.

Iran initially came to the negotiating table because of the pain caused by economic sanctions imposed by Congress, a series of U.S. presidents, several foreign nations and the United Nations.

Proponents of the deal say that it has worked — that Iran has kept its promises to halt the nuclear weapons program and to allow international vetting of its energy capabilities.

But Trump campaigned on rewriting the pact, and, during a speech to the U.N. last year, he called it a major “embarrassment” and “one of the worst and most one-sided transactions the United States has ever entered into.”

Until Tuesday, though, Trump had taken a more measured approach to the deal. Last October, in declining to certify that Iran had lived up to its end of the bargain, Trump kicked a decision on whether to re-impose U.S. sanctions to Congress — which did not act.

Trump pushed the call to Congress, even though he always has had the power to re-impose sanctions through executive action and perhaps because he knew it was unlikely Congress would be capable of agreeing to anything.

Back then, his national security adviser was H.R. McMaster, who was considered a moderate on Iran. Now, though — with leading Iran critic John Bolton newly installed in McMaster’s old role — Trump has decided to take on Tehran.

That has worried America’s European allies, Democrats and some Republican in Congress.

Even if Iran doesn’t kick-start its nuclear weapons program, its leaders could banish inspectors.

“When our country, even if it’s just the president, makes an agreement with our allies with another country, we ought to be very careful about changing that,” Sen. Lamar Alexander, R-Tenn., who opposed the deal in the first place, said on Hugh Hewitt’s radio show Monday. Though the deal has “a lot of flaws,” he said, it does give “a window into what Iran is doing and limits their development of nuclear weapons.”

British and French officials urged Trump not to disrupt the deal in advance of Tuesday’s announcement.