In his September speech to the U.N. General Assembly, President Trump expressed disdain for the Iran deal created under President Obama and noted that the world hadn’t “heard the last of it.”

President Trump, who has called the Iran nuclear agreement the “worst deal ever,” plans to announce Friday his refusal to re-certify Iran’s compliance with the 2015 deal. Here’s what you need to know:

What is the Iran nuclear agreement? 

In 2015, President Barack Obama and the leaders of the United Kingdom, France, Russia, China, Germany and the European Union reached an agreement with Iran that required it to freeze its nuclear program for at least a decade. Obama said he  wanted to make sure Iran doesn’t develop nuclear weapons, though the Iranian government said its research is for peaceful uses.

As part of the agreement, the U.S. and its allies agreed to lift international sanctions that have strangled Iran’s economy. That has given oil-rich Iran billions of fresh revenue.

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U.N. inspectors say Iran is complying with the deal. But critics, including Trump, complain that Iran is using its new revenue to fund militant groups in the Middle East and develop ballistic missiles, which are not covered by the deal.

What does Trump’s ‘decertification’ announcement mean? 

Congress requires the president to certify Iran’s compliance with the agreement every three months. Decertification does not kill the deal outright. 

Trump will formally announce his decision to decertify the Iran deal, and he will also ask Congress not to reimpose…