BBC
The final column of about 100 armoured vehicles carrying 500 soldiers crossed the southern Iraqi desert overnight.
At the peak of the operation there were 170,000 US troops and more than 500 bases in Iraq.
Nearly 4,500 US soldiers and tens of thousands of Iraqis have died since the US-led campaign began in 2003.
The operation has cost Washington nearly $1 trillion (£643bn).
"Part of history, you know - we're the last ones out."
As the last of the armoured vehicles crossed the border, a gate was closed behind them and US and Kuwaiti soldiers gathered there to shake hands and pose for pictures.
The only US military presence left in Iraq now is 157 soldiers responsible for training at the US embassy, as well as a small contingent of marines protecting the diplomatic mission.
The low-key US exit was in stark contact to the blaze of aerial bombardment Washington unleashed against Saddam Hussein in 2003.
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US President Barack Obama marked the end of the war earlier in the week, meeting Iraqi Prime Minister Nouri al-Maliki.
He announced in October that all US troops would leave Iraq by the end of 2011, a date previously agreed by former President George W Bush in 2008.
In a recent speech at Fort Bragg in North Carolina, President Obama paid tribute to the soldiers who had served in Iraq.
He acknowledged that the war had been controversial, but told returning troops they were leaving behind "a sovereign, stable and self-reliant Iraq".
However, correspondents say there are concerns in Washington that Iraq lacks robust political structures or an ability to defend its borders.
There are also fears that Iraq could be plunged back into sectarian bloodletting, or be unduly influenced by Iran.
Washington had wanted to keep a small training and counter-terrorism presence in Iraq, but US officials were unable to strike a deal with Baghdad on legal issues including immunity for troops.
The exit of US troops leaves behind a fragile democracy in Iraq
The last convoy of US troops to leave Iraq has entered Kuwait, nearly nine years after the invasion that toppled Saddam Hussein.
At the peak of the operation there were 170,000 US troops and more than 500 bases in Iraq.
Nearly 4,500 US soldiers and tens of thousands of Iraqis have died since the US-led campaign began in 2003.
The operation has cost Washington nearly $1 trillion (£643bn).
Continue reading the main story 
US troops have trained up Iraqi security forces which, if they stick together, can arguably contain the internal security situation, still stubbornly jammed at a level of violence which kills on average around 350 people every month.
But security has to be rooted in political stability, and that's only one of many challenges immediately facing Iraq.
Even as the final US troops were heading for the border, a political crisis was erupting in Baghdad, with deputies from Ayyad Allawi's Iraqiyya block pulling out of parliament.
There is turmoil in two mainly Sunni provinces, which want to declare themselves autonomous regions like the Kurds in the north. There's also a widespread conviction that with the Americans gone, Iranian influence will spread.
While most Iraqis believe it was high time for the Americans to go, many are deeply worried about the challenges that lie ahead.
Analysis
But security has to be rooted in political stability, and that's only one of many challenges immediately facing Iraq.
Even as the final US troops were heading for the border, a political crisis was erupting in Baghdad, with deputies from Ayyad Allawi's Iraqiyya block pulling out of parliament.
There is turmoil in two mainly Sunni provinces, which want to declare themselves autonomous regions like the Kurds in the north. There's also a widespread conviction that with the Americans gone, Iranian influence will spread.
While most Iraqis believe it was high time for the Americans to go, many are deeply worried about the challenges that lie ahead.
US forces ended combat missions in Iraq in 2010 and had already handed over much of their security role.
"(It's) a good feeling... knowing this is going to be the last mission out of here," said Private First Class Martin Lamb, part of the final "tactical road march" out of Iraq."Part of history, you know - we're the last ones out."
As the last of the armoured vehicles crossed the border, a gate was closed behind them and US and Kuwaiti soldiers gathered there to shake hands and pose for pictures.
The only US military presence left in Iraq now is 157 soldiers responsible for training at the US embassy, as well as a small contingent of marines protecting the diplomatic mission.
The low-key US exit was in stark contact to the blaze of aerial bombardment Washington unleashed against Saddam Hussein in 2003.
Tribute
US President Barack Obama marked the end of the war earlier in the week, meeting Iraqi Prime Minister Nouri al-Maliki.
He announced in October that all US troops would leave Iraq by the end of 2011, a date previously agreed by former President George W Bush in 2008.
The US troops left Iraq for the last time, crossing into Kuwait
He acknowledged that the war had been controversial, but told returning troops they were leaving behind "a sovereign, stable and self-reliant Iraq".
However, correspondents say there are concerns in Washington that Iraq lacks robust political structures or an ability to defend its borders.
There are also fears that Iraq could be plunged back into sectarian bloodletting, or be unduly influenced by Iran.
Washington had wanted to keep a small training and counter-terrorism presence in Iraq, but US officials were unable to strike a deal with Baghdad on legal issues including immunity for troops.
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