What is the Iran Nuclear Deal? 

The deal, formally known as the Joint Comprehensive Plan of Action, is an agreement reached between Iran and the five permanent members of the U.N. Security Council, along with the European Union and Germany in October 2015.

The deal requires Iran to:

Eliminate its stockpile of medium-enriched uranium
Reduce its stockpile of low-enriched uranium by 89 percent
Reduce by two-thirds the number of centrifuges capable of enriching uranium
Limit uranium-enriching to a single facility with first-generation centrifuges

In return:

The U.N. would lift sanctions related to the nuclear program
The European Union and the United States would lift some sanctions
Frozen assets worth more than $100 billion would be released to Tehran

Why does it need to be certified?

The U.S. Congress passed a bill that requires the president to certify Iran’s compliance with the deal to the congress every 90 days. 

Will Trump certify the deal? 

Trump has certified the deal twice since taking office in January. But the president, who has called it the “worst deal ever,” has suggested he won’t do it the third time. The deadline for the next certification is Oct. 15.

What happens if he doesn’t certify it? 

If Trump does not certify the deal, Congress will have 60 days to reinstate the old sanctions on Iran.
If Congress decides to, instead, impose new sanctions, that would be in violation of the U.S. commitment to the agreement and the deal would be broken.
If Congress decides not to act either way, the deal will stay in place for the next 90 days.
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