A brief history of the long-strained relations between the United States and Iran:
1953: A CIA-backed coup overthrows Iranian Prime Minister Mohamed Mossadegh, restores shah to power. The U.S. provides the increasingly autocratic shah hundreds of millions of dollars over the next quarter-century.
1979: Iranians overthrow the shah. Ayatollah Ruhollah Khomeini returns from exile, seizes power and declares the U.S. the "Great Satan." Militants storm the U.S. Embassy and hold 52 Americans hostage for 444 days. The United States ends diplomatic relations with Iran.
1980-88: U.S. supports Saddam Hussein in the Iran-Iraq war. Some 1.5 million people are killed.
1983: Iranian-backed Hezbollah is blamed for the bombings of the U.S. Embassy in Lebanon and of the Beirut barracks of the U.S. Marine Corps, which killed 258 Americans.
1986: The Reagan administration is exposed for covertly selling arms to Iran and using the proceeds to bankroll a secret war in Central America.
1987-88: U.S. and Iranian forces clash in Persian Gulf.
1998: The U.S. mistakenly downs an Iranian passenger jet flying above the Strait of Hormuz, killing 290 people. Iran and Iraq reach a cease-fire.
1990s: Iran is accused of supporting a series of Hamas and Hezbollah terrorist attacks around the world.
1995: President Bill Clinton imposes far-reaching oil and trade sanctions on Iran.
1997: Iran elects reformist President Mohammed Khatami as president. The U.S. scales back some sanctions.
2001: After the 9/11 terrorist attacks, U.S. and Iran coordinate on action against the Taliban and aid to Afghanistan.
2002: President George W. Bush includes Iran in his "Axis of Evil." Washington releases information about Iran's nuclear program.
2003: After ousting Saddam Hussein and occupying Iraq, U.S. accuses Iran of helping Shiite militants kill American soldiers.
2005: Hard-line conservative Mahmoud Ahmadinejad becomes Iran's president, issues a series of provocative statements against the U.S. and Israel.
2006-2010_The U.S. succeeds in getting four rounds of U.N. sanctions passed against Iran. They demand Tehran stop enriching uranium and exporting weapons, and they set banking, trade and travel restrictions.
2009: Obama takes office promising engagement with Iran. Months later, Ahmadinejad gets another term as president after a contested vote and violent, postelection crackdown. U.S. and Israel covertly sabotage Iran's nuclear program.
2011: U.S. and Iran support opposing sides in Syria's civil war. After violence breaks out, Tehran actively helps Syrian President Bashar Assad, while Washington slowly expands aid to the rebels.
2012: U.S. begins working with countries around the world to reduce oil purchases from Iran. A year later, Iran's exports drop by half and its economy is in tatters.
2012-13_Several rounds of nuclear negotiations between Iran and world powers fail to make progress.
2013: Hasan Rouhani assumes the Iranian presidency, promising a new course of moderation.