Relations between Iran and the United States - once strong allies - have often been mistrustful and sometimes openly hostile since Iran's 1979 revolution.
Here are some key dates:
1953 - The U.S. Central Intelligence Agency helps orchestrate the overthrow of Iran's popular Prime Minister Mohammed Mossadegh, restoring to power Shah Mohammed Reza Pahlavi.
1967 - The United States provides Iran with a nuclear reactor and enriched uranium fuel. Iran signs the Nuclear Non-Proliferation Treaty, allowing it a civilian nuclear programme but not a military one, in 1968.
1972 - U.S. President Richard Nixon visits Tehran to strengthen security relations between the countries - part of a policy to make Iran and Saudi Arabia "twin pillars" of the U.S. strategy to contain Soviet influence in the Middle East.
1979 - Iran's Islamic Revolution forces the shah to flee. Ayatollah Ruhollah Khomeini returns from exile and becomes Supreme Leader. Students seize the U.S. Embassy in Tehran and hold staff hostage.
1980 - The United States cuts diplomatic ties with Iran, seizes Iranian assets and bans most trade with it. A hostage rescue mission ordered by President Jimmy Carter fails. The hostages are released minutes after Carter steps down.
1983 - Lebanon's Hezbollah group, founded and backed by Iran, is accused by the U.S. of bomb attacks on its Beirut embassy and marine barracks that kill about 300 people, mostly Americans. Hezbollah has said other groups were responsible.
1984 - The United States restores ties with Iraq, giving it diplomatic backing in a war against Iran.
1986 - U.S. President Ronald Reagan reveals secret arms deal with Tehran in violation of an arms embargo.
1988 - The U.S. Navy destroys two Iranian oil platforms and sinks a frigate in retaliation for damage to a U.S. vessel that hit an Iranian mine. U.S. warship Vincennes mistakenly shoots down an Iranian passenger plane over the Gulf, killing all 290 aboard.
2002 - President George W. Bush declares Iran, Iraq and North Korea an "axis of evil". U.S. officials accuse Tehran of operating a secret nuclear weapons program.
2003 - Following the U.S. invasion of Iraq, Iran-backed Shi'ite Muslim militias gain wide sway in parts of the country and stage attacks on U.S. forces.
2011 - The FBI says it has uncovered an Iranian plot to assassinate the Saudi ambassador to Washington. Iran denies any involvement.
2012 - U.S. law gives U.S. President Barack Obama, who has offered to extend a hand if Tehran "unclenches its fist", power to sanction foreign banks if they fail to significantly reduce imports of Iranian oil, leading to an economic downturn in Iran.
2013 - Hassan Rouhani is elected Iran's president on a platform of improving its relations with the world and boosting the economy.
2015 - Iran and six major powers, including the United States, agree to a nuclear deal curbing Iran's nuclear work in return for limited sanctions relief.
2016 - Iran releases 10 U.S. sailors who ended up in Iranian territorial waters. The United States and Iran swap prisoners.
2018 - U.S. President Donald Trump withdraws from the nuclear deal and reimposes economic sanctions on Iran.
2019 - Oil tankers are attacked in the Gulf in May and June. The United States blames Iran, a charge Tehran denies.
2020 - The United States kills Islamic Revolutionary Guard Corps Quds Force head Qassem Soleimani with a strike in Iraq. Iran strikes back with missile attacks on Iraqi bases housing American troops, injuring about 100.
2022 - Major protests grip Iran. The United States imposes sanctions on entities it says are involved in the crackdown.
2023 - Iran releases five imprisoned Iranian-American dual citizens in return for a sanctions waiver. Washington releases five detained Iranians days later.
On October 7, Iran-backed Palestinian militant group Hamas attacks U.S.-ally Israel, killing 1,200 people and seizing around 250 hostages, prompting an Israeli military campaign in Gaza that Palestinian health authorities say has killed more than 50,000 people.
Iran-backed Hezbollah starts firing on Israel in what it calls solidarity with Gaza. Yemen's Iran-backed Houthi group attacks Red Sea shipping and fires drones at Israel in support of Hamas.
2024 - Israeli attacks on Islamic Revolutionary Guard Corps personnel in Syria and the killing of Hezbollah leader Hassan Nasrallah prompt two direct Iranian attacks on Israel. The United States helps shoot down Iranian missiles and drones.
2025 - Trump threatens to bomb Iran if it does not come to an agreement with Washington over its nuclear programme. Iran's Supreme Leader Ayatollah Ali Khamenei says any U.S. attack on his country would prompt "a strong reciprocal blow".
