"Since the Islamic Revolution in Iran in 1979, the United States and Iran have been at odds politically and diplomatically. During the 1980s and 1990s, U.S. officials identified Iran's support for militant Middle East groups as the primary threat posed by Iran to U.S. interests and allies. Iran's nuclear program took precedence in U.S. policy after 2002 as the potential for Iran to develop a nuclear weapon increased. Beginning in 2010, the United States orchestrated broad international economic pressure on Iran to persuade it to agree to strict limits on the program-- pressure that contributed to the June 2013 election of the relatively moderate Hassan Rouhani as president of Iran and the eventual negotiation of a 'Joint Comprehensive Plan of Action' (JCPOA). The JCPOA, which began formal implementation on January 16, 2016, exchanged broad sanctions relief for nuclear program limits intended to ensure that Iran would require at least a year to produce a nuclear weapon after a decision to do so."
CRS Report for Congress, RL32048
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html