Iranian surface-to-surface missiles.

President Joe Biden entered office promising to return the U.S. to the Iran nuclear deal, formally known as the Joint Comprehensive Plan of Action, or the JCPOA. But doing so has proven tricky for Biden’s administration, in part because of the complex politics surrounding the deal in both Washington and Tehran, but also because of the tense relations between the two countries, which soured significantly under Biden’s predecessor, Donald Trump.