The assassination, which comes after a long history of US military intervention on Iraqi soil, represents one of the most extreme escalations of tension between the US and the Middle East in recent years.
Read more: Who was Qassem Soleimani, the powerful commander of Iran's elite Quds force?
Below are the key moments in relations between the United States and Iraq since the 2003 US invasion.
US invasion
On 20 March, 2003, a US-led invasion of Saddam Hussein's Iraq is launched after claims his regime is harbouring weapons of mass destruction. US forces take control of Baghdad the following month.
US President George W. Bush announces the end of major combat operations on 1 May, but says the "war on terrorism" continues.
On 2 October, a US report says no weapons of mass destruction have been found in Iraq.
Saddam is captured on 13 December near Tikrit, north of Baghdad. He is hanged three years later.
The broadcast in April 2004 of images of torture and other illegal abuse of Iraqi prisoners at the Abu Ghraib US military prison shocks the world.
Power is transferred to an interim government in June.
Insurgency and sectarian conflict
In November 2004, more than 10,000 American and 2,000 Iraqi soldiers attack the flashpoint Sunni city of Fallujah which had become a symbol of resistance to the foreign presence after the lynching of four Americans in March.
In February 2006, Al-Qaeda-linked jihadists blow up a Shia shrine in Samarra, sparking a wave of sectarian killings which leaves tens of thousands dead and lasts until 2008.
In January 2007, Bush boosts the number of troops to 165,000, saying the surge is needed to help Iraq's embattled government.
America takes its boots off the ground
On 18 December, 2011, US president Barack Obama, who had deeply opposed the war in Iraq, withdraws the last American soldiers, leaving the country mired in a severe political crisis.
Between 2003 and 2011 more than 100,000 civilians were killed, according to the Iraq Body Count database. The United States lost nearly 4,500 troops.
Fighting the Islamic State
In January 2014 jihadists from the Islamic State of Iraq and the Levant (ISIL), later known as the Islamic State (IS) group, capture Fallujah and parts of Ramadi city. In June they seize Mosul and by the end of 2014 hold one-third of Iraq.
The United States intervenes directly in Iraq, bombarding jihadist positions which threaten Iraqi Kurdistan and thousands of members of the Christian and Yazidi minorities.
With the help of the US-led coalition, Iraqi forces drive IS from all urban centres.
In December 2017 Iraq declares the "end of the war" against IS.
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| Iraqis gather in Baghdad's Tahrir Square on 10 December, 2017, to celebrate the end of the three-year war against the Islamic State (IS) group. [Getty] |
Iran steps in
Iran, which backs the Hashd al-Shaabi Iraqi paramilitary group that played a key role in fighting IS, becomes an influential ally and major trading partner of Baghdad.
Mass demonstrations erupt in Iraq on 1 October, 2019, against unemployment, corruption and poor public services, but also against interference by foreign powers, notably Iran.
Since October, at least 12 attacks target Iraqi military bases where US soldiers or diplomats are deployed.
Some 5,200 US soldiers are currently deployed in Iraq as part of the anti-IS coalition.
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| Iraqi security forces confront protestors as they demonstrate against the government and the lack of basic services, on 4 January, 2019 in the southern city of Basra. [Getty] |
US embassy siege
In late December 2019, a rocket attack kills an American contractor.
In response, US airstrikes target bases belonging to the Hezbollah brigades - one of the most radical factions of the Hashd - near the Iraqi-Syrian border, killing at least 25 Iraqi fighters.
Iraq's caretaker government warns the strikes could affect ties with Washington
An outraged pro-Iran mob lays siege to the US embassy in Baghdad on New Year's Day, in an unprecedented attack on the American mission.
Elite Iraqi troops secure the embassy the following day.
Iran commander assassinated
On 3 January, in the most significant escalation yet, US President Donald Trump orders strikes on Baghdad's international airport, killing top Iranian commander Qassem Soleimani and the deputy head of the Hashd, Abu Mahdi al-Muhandis.
The Pentagon says Soleimani had been "actively developing plans to attack American diplomats and service members in Iraq and throughout the region".
Iran's supreme leader Ayatollah Ali Khamenei vows "severe revenge".