Iran, known as Persia until 1935, became an Islamic republic in 1979 after the ruling monarchy was overthrown and Shah Mohammad Reza Pahlavi was forced into exile, according to the CIA World Factbook. Conservative clerical forces led by Ayatollah Ruhollah Khomeini established a theocratic system of government with ultimate political authority vested in a religious scholar known as the supreme leader, who is accountable only to the Assembly of Experts — an elected, 88-member body of clerics. U.S.-Iran relations became strained when Iranian students seized the U.S. Embassy in Tehran in November 1979 and held embassy personnel hostage until mid-January 1981. The U.S. cut off diplomatic relations with Iran in April 1980. From 1980 to 1988, Iran fought a bloody, indecisive war with Iraq that eventually expanded into the Persian Gulf and led to clashes between U.S. Navy and Iranian military forces. Iran has been designated a state sponsor of terrorism since 1984.

Nuclear program beginnings 1967 — Iran takes possession of its Tehran Research Reactor under America’s “Atoms for Peace” program.

1979 — The terminally ill shah flees as popular protests against him surge.

2002 — Western intelligence services and an Iranian opposition group reveal the nation's secret Natanz nuclear enrichment facility.

2003 — Britain, France and Germany engage Iran in nuclear negotiations.