What are the Iran nuclear talks about?

Negotiations to bring the United States back into a landmark nuclear deal with Iran resumed Thursday in Vienna amid signs of progress — but also under the shadow of an attack this week on Iran’s main nuclear facility.

In 2015, Iran signed an agreement with the U.S., Russia, China, Germany, France and Britain that was intended to set limits on Tehran’s nuclear program in order to block it from building a nuclear weapon — something it insists it doesn’t want to do. In exchange, Iran received relief from sanctions that those powers had imposed, including on its exports of oil and access to the global banking system. Iran was allowed to continue to pursue its nuclear program for civilian purposes, with strict limits on how much uranium it could enrich, the purity it could enrich it to and other measures.