US forces make up the lion's share of coalition firepower in Iraq although numbers have dropped slightly since the peak of the surge in October. There were 157,000 troops in the country in February 2008. The number of United States military personnel killed in Iraq since the US-led invasion in 2003 passed the 4,000 mark in late March 2008.
Air strikes over Baghdad signalled the start of the US-led invasion of Iraq. President Bush said the invasion was undertaken because Saddam Hussein's government possessed weapons of mass destruction and supported terrorism. Within three weeks Baghdad had fallen and the country was under the control of US-led coalition.
1 May 2003: Major combat operations ‘over'
Despite President Bush's declaration that major combat operations in Iraq had ended, attacks on coalition forces were developing into a fully fledged insurgency. Law and order had never been fully restored after Saddam Hussein's defeat. Much of the initial violence was led by Sunni fighters, opposed to the invasion and the establishment of a new government.
28 June 2004: US hands over power
The US-led coalition returned sovereignty to Iraq, allowing the appointed interim Prime Minister Iyad Allawi to assume power. A cabinet was also sworn into office. The establishment of a new government was conducted in the face of mounting resistance from insurgents and growing US casualties.
September 2004: US fatalities reach 1,000
As the conflict continued, so the sophistication of insurgent attacks grew. Car bombs and improvised explosive devices placed at roadsides took an increasing toll on American soldiers.
7 November 2004: Assault on Falluja
Falluja, a stronghold of the Sunni insurgency west of Baghdad, was the scene of fierce fighting for several weeks at the end of 2004. The joint assault by the US and the Iraqi armies displaced most of the city's population and left many of its buildings destroyed.
30 January 2005: Elections for multi-party assembly
Iraqi voters went to the polls to choose the 275 members of their new national assembly. The United Iraqi Alliance took the most votes, but fell short of a majority. In some Sunni areas turnout was as low as 2%. The new assembly was given the task of drawing up Iraq's constitution.
October 2005: US fatalities reach 2,000
In October the number of dead American service personnel reached another landmark. Most of those killed were in their 20s. Roadside bombs and hostile fire were the two main causes of death, and the regions of Baghdad and Anbar accounted for over 50% of US casualties. In the same month Iraqi voters approved a new constitution which aspired to set up an Islamic federal democracy.
20 Jan 2006: Nationwide elections
Provisional figures gave the Shia-led United Iraqi Alliance a victory in Iraq's first elections since Saddam Hussein was removed from power.
31 Dec 2006: US fatalities reach 3,000
Texan Dustin R. Donica became the 3,000th US soldier to die in Iraq. He was killed by small arms fire in Baghdad. His death came as President Bush considered increasing American troop levels, with the aim of stabilising an increasingly bloody situation.
February 2007: Launch of security surge
The US launched a joint US-Iraqi offensive aimed at restoring security in Baghdad in February. Extra troops were sent to the area in and around the capital as part of the security drive, or surge, which followed escalating violence.
August 2007, Mehdi Army announces 'freeze'
The radical Shia cleric Moqtada Sadr announces a freeze on the activities of his Mehdi Army militia for six months.
December 2007: Security improves after surge
Top US commander Gen David Petraeus says the troop surge has coincided with a "remarkable" improvement in security in Iraq. The plan is to begin reducing US troop
numbers by about 20,000 by July 2008. Any further reduction in numbers will depend on conditions on the ground.