Six world powers have worked out the details of a six-month nuclear deal with Iran that will require the country to begin dismantling its nuclear program starting on Jan. 20.

An agreement was reached in November that would constrain much of Iran’s program in return for billions of dollars in relief from crippling economic sanctions, but details for carrying out the deal had not been reached until Sunday, the New York Timesreports.

On Jan. 20, Iran will begin eliminating its stockpile of higher levels of enriched uranium and dismantling some of its enrichment infrastructure, President Obama said, while adding “Imposing additional sanctions now will only risk derailing our efforts to resolve this issue peacefully, and I will veto any legislation enacting new sanctions during the negotiation.”

The current agreement is temporary, and negotiators hope it will be a precursor to a broader agreement requiring Iran to substantially undo…