Incoming national security adviser Jake Sullivan told CNN’s Fareed Zakaria on Sunday that after returning to the deal, known as the Joint Comprehensive Plan of Action (JCPOA), Biden will pursue follow-on negotiations to constrain Iran’s regional behavior.

Iran’s ballistic missile program, Sullivan said, “has to be on the table” in those negotiations.

The framework might appear to be in place, as President Hassan Rouhani says Iran is also prepared to return to compliance if the U.S. lifts its sanctions. But today’s enrichment announcement underscores just how thorny the process will be.

If Iran enriches significant quantities of uranium to 20%, its nuclear breakout time will become “very, very small,” says Ernest Moniz, who played a key role in negotiating the 2015 deal as Barack Obama’s energy secretary. “The key question is how much they make.”