U.S. President Donald Trump’s administration said late Saturday all United Nations sanctions on Iran have been restored and a conventional arms embargo on the country will no longer expire in mid-October.

In addition, the U.S. will sanction more than two dozen people and entities involved in Iran’s nuclear, missile and conventional arms programs, a senior U.S. official said, putting teeth behind the UN sanctions.

But 13 of the 15 UN Security Council members, including long-time U.S. allies, say Washington’s move is void and diplomats say few countries are likely to reimpose the measures, which were lifted under a 2015 deal between world powers and Iran that aimed to stop Tehran developing nuclear weapons.

“The return of sanctions today is a step toward international peace and security,” Secretary of State Mike Pompeo said in a statement. “In the coming days, the United States will announce a range of additional measures to strengthen implementation of UN sanctions and hold violators accountable.”

Here is a look at the events leading to this showdown and an explanation of what could happen next:

Why Is the Arms Embargo on Iran Expiring?

The Security Council imposed an arms embargo on Iran in 2007.

The embargo is due to expire on October 18, as agreed under the nuclear deal among Iran, Russia, China, Germany, Britain, France and the United States that seeks to prevent Tehran from developing nuclear weapons in return for sanctions relief. It is enshrined in a 2015 Security Council resolution.

In 2018, Mr. Trump quit the accord reached under his predecessor Barack Obama. The United States failed last month in a bid to extend the Iran embargo at the Security Council.

What Does This Mean for the 2015 Nuclear Deal?

The remaining parties to the nuclear deal have said they are committed to maintaining the agreement. Iran has said it would remain in place despite the U.S. move at the United Nations.

Britain, France and Germany told the UN Security Council on Friday that UN sanctions relief for Iran would continue beyond September 20.

“We have worked tirelessly to preserve the nuclear agreement and remain committed to do so,” the UN envoys for the three countries said in a letter to the Council.

What Sanctions Would “Snapback”?

A return of UN sanctions, a so-called snapback, would require Iran to suspend all nuclear enrichment-related and reprocessing activities, including research and development, and ban imports of anything that could contribute to those activities or to development of nuclear weapon delivery systems.

It would reimpose the arms embargo, ban Iran from developing ballistic missiles capable of delivering nuclear weapons and bring back targeted sanctions on dozens of individuals and entities. Countries also would be urged to inspect shipments to and from Iran and authorized to seize any banned cargo.

What Led to This Showdown?

The United States submitted a complaint about Iran breaching the nuclear deal to the Security Council last month.

In response to the U.S. quitting the accord and imposing unilateral sanctions in a bid to get Iran to negotiate a new deal, Tehran has breached central limits of the pact, including on its stock of enriched uranium.

Under a 2015 UN Security Council resolution enshrining the nuclear deal, the United States says that it triggered a 30-day process leading to a snapback of all UN sanctions on Iran. Washington argues that while it quit the nuclear deal in 2018, the 2015 resolution still names it as a participant.

Under the sanctions snapback process, if a Security Council resolution to extend sanctions relief on Iran is not adopted within the 30 days, then UN sanctions are supposed to be reimposed. No such resolution has been put forward for a vote.

Thirteen of the 15 council members expressed their opposition, arguing that Washington’s move is void given it is using a process agreed under the nuclear deal it is no longer a party to.

What Has the U.S. Done Now?

The United States slapped additional sanctions on Iran on Monday after the Trump administration’s unilateral weekend declaration that all UN penalties eased under the 2015 nuclear deal had been restored.

“The United States has now restored U.N. sanctions on Iran,” Mr. Trump said in a statement issued shortly after he signed an executive order spelling out how the U.S. will enforce the “snapback” of the sanctions. “My actions today send a clear message to the Iranian regime and those in the international community who refuse to stand up to Iran.”

The sanctions include freezes on any assets those targeted may have in U.S. jurisdictions, bar Americans from doing business with them and, perhaps most importantly, open up foreign governments. companies and individuals to U.S. penalties if they engage in transactions with them.

This article contains information from Reuters and The Associated Press.