A wide range of armed groups are operating in Iraq, feeding into violence which has prompted fears of civil war. Much insurgent and militia activity is shadowy and difficult to trace, but here is a guide to the main players.
INSURGENT GROUPS: BACKGROUND
Since the downfall of Saddam Hussein, insurgent groups have targeted coalition forces and anyone working alongside the US in the rebuilding of Iraq.
Insurgents have increasingly also turned on Shia targets - both those linked to the Shia-dominated Iraqi government and civilian targets such as Shia shrines and festivals.
Tracking the insurgency's size and make-up is notoriously difficult, with groups constantly appearing and disappearing, and allegiances shifting.
The insurgency is mainly Sunni, but draws its membership from diverse backgrounds.
Fighters range from former figures in Saddam Hussein's Baath party to Sunni nationalists fearing Shia domination and foreign Islamist fighters who see Iraq as an arena for a global struggle against the West.
The incentives driving individual insurgents are equally disparate - from religious zeal to economic gain, nationalist feeling and anger at the loss of loved ones to the conflict.
Virtually all insurgent groups share the goal of attacking US forces, but other goals vary - with some elements apparently aiming to foment civil war.
Estimates of the number of insurgents are impossible to confirm.
By 2006, US military estimates ranged from 8,000 to 20,000, although Iraqi intelligence officials have issued figures as high as 40,000 fighters plus another 160,000 supporters.
Fighters have been found among the insurgents from countries including Syria, Saudi Arabia, Yemen and Sudan.
Foreign nationals are widely considered to account for less than 10% of the insurgency, but their role is high profile.
Some Sunnis have also formed informal militias, which operate as private defence forces in certain neighbourhoods where Shia militias are thought likely - or known - to carry out attacks.
AL-QAEDA IN IRAQ
Al-Qaeda of Jihad Organisation in the Land of the Two Rivers is the country's most prominent insurgent group, blamed for many of the bloodiest bombings and beheadings.
Zarqawi was blamed for many bloody attacks
It was led by the Jordanian militant, Abu Musab al-Zarqawi, until he was killed by a US air strike in June 2006.
He emerged in Iraq as the alleged head of the Tawhid and Jihad group, which was blamed for some of the biggest early insurgent attacks.
These included the truck bombing that killed 23 at the UN headquarters in Baghdad in August 2003 and the blast in Najaf 10 days later that killed a senior Shia cleric and more than 85 others.
Tawhid and Jihad was also known for the brutal beheadings of foreign hostages, which were posted on the internet in video footage attributed to the group.
An internet statement in 2004 claimed that the group had joined Osama Bin Laden's al-Qaeda network, although the depth of the links is not clear.
Al-Qaeda in Iraq's hallmarks include synchronised bomb attacks, as well as the abduction and murder of foreign hostages.
The bombs have had a range of targets - from US military personnel to Iraq's fledgling security forces and its Shia community. Shia Muslims have been derided as apostates in messages attributed to Zarqawi.
Igniting sectarian conflict is central to al-Qaeda's strategy in Iraq, according to a letter purportedly authored by Zarqawi and released by the US military in early 2004.
Foreign "jihadi" fighters are widely thought to play a key role in the group, although some analysts say it may have also have a considerable Iraqi membership.
Shortly after Zarqawi's death, al-Qaeda in Iraq named a new leader, Abu-Hamzah al-Muhajir, thought to be a pseudonym.
Uncertainly surrounds the new leader, although the US military released a picture of a man it named as Abu Ayyub al-Masri and said was an Egyptian militant based in Baghdad.
MUJAHIDEEN SHURA COUNCIL
In early 2006, al-Qaeda in Iraq posted an internet statement saying it had joined five other insurgent groups in Iraq to form a new umbrella organisation, the Mujahideen Shura Council.
Two of these groups were known - the Victorious Sect Army and the Islamic Jihad Brigade, while three were apparently new groups.
The Mujahideen Shura Council issues statements and posts videos on a website - including a video showing the executions of two Russian hostages in June 2006.
Analysts say moves to build co-operation and a unified strategy among the disparate insurgent groups have had some success.
A section of the insurgency comprising former elements of Saddam Hussein's regime, Baath party supporters, former Iraqi soldiers and secular Sunnis is often referred to as "Sunni nationalists".
Some Iraqi cities have been dominated and run by insurgents
Analysts believe that in the wake of the US-led invasion, some former regime figures provided the nascent insurgency with access to regime funds and weapons caches.
In September 2005, an Iraqi court convicted a nephew of the deposed leader of funding insurgents.
Commentators have also blamed much of the violence on the decision by former US governor Paul Bremer to disband the Iraqi army in 2003, without disarming it.
US forces have faced their greatest challenges in areas of central Iraq - such as the city of Falluja - that had a strong tradition of military service.
Since late 2005, the US has said it is trying to drive a wedge between the more extreme Islamist groups and the more secular and moderate nationalists.
Sunni insurgent groups were split over participation in elections in December 2005, although support from some boosted significant Sunni turnout and thus Sunni influence on the new government.
But a report by Anthony Cordesman, of the Center for Strategic and International Studies, said even Sunni leaders who were participating in - rather than attacking - the political process, were forming their own forces to counter the existing Shia militias.
Ansar al-Islam is a radical Sunni Muslim group with its base in mountainous northern Iraq.
It draws its recruits from Kurds who oppose the US-backed Patriotic Union of Kurdistan party, according to US intelligence website globalsecurity.org.
The group, which is also known as Ansar al-Sunna, suffered a severe setback during in early 2003, losing many bases to the US bombing campaign.
In February 2004, it claimed responsibility for simultaneous suicide bomb attacks on the offices of the two US-backed mainstream Kurdish political parties.
The group's alleged leader, Mullah Krekar, has lived in Norway as a refugee since 1991.
The US has said Ansar al-Islam has ties to al-Qaeda - it has also accused Iran of giving support to the group.
SHIA MILITIAS: BACKGROUND
Some Iraqi political parties have armed wings, despite US pressure to disband militias.
A growing trend of sectarian killings in Baghdad and other mixed Sunni and Shia areas of the country has prompted fears of civil war.
Groups of corpses, typically with hands bound and gunshot wounds to the head, sometimes bearing signs of torture, have regularly been found.
In some cases gunmen, sometimes dressed in the uniforms of government security forces, abduct victims or pull them from cars at checkpoints.
There are widespread suspicions that militias linked to two key Shia parties are involved in targeting Sunnis.
While the allegations have not been proven, these militias are becoming increasingly prominent as sectarian divisions grow.
The US ambassador to Iraq, Zalmay Khalilzad, has described such groups as "the infrastructure of civil war".
Separately from the main militias, some Shia have also formed informal defence forces which provide security for local neighbourhoods, with armed guards carrying out patrols and manning roadblocks.
The Mehdi Army is led by Moqtada Sadr, a young, radical Shia cleric.
The Mehdi Army has taken part in two uprisings against US forces
His brand of Shia nationalism, opposition to the US presence in Iraq and hostility toward the powerful established Shia political parties has proved popular among poor, disenfranchised Shia.
Mr Sadr's key stronghold is the slum district of Sadr City, named after his father, a revered cleric murdered by Saddam Hussein's security forces.
The US has accused Iran of providing funding and support to the Mehdi Army.
Mehdi fighters staged uprisings against US-led forces in April and August 2004, but Mr Sadr has since become involved in the political process and holds 30 seats in the Shia bloc that dominates parliament.
As insurgent attacks have increasingly targeted Shia areas, the Mehdi Army has become one of the major armed forces on the ground in Baghdad, controlling - and protecting - predominantly Shia areas.
Sunnis have accused its members of carrying out sectarian killings, although Mr Sadr denies the accusations.
The Badr Brigade - sometimes called the Badr Organisation - is the armed wing of the largest Shia party in Iraq, the Supreme Council for the Islamic Revolution (Sciri) in Iraq.
Sciri opposed Saddam Hussein from Iran for many years.
The Badr Brigade waged a low-level war of ambushes, sabotage, and assassinations against the regime, using undercover cells in Iraq and bases in Iran.
Sciri's leader, Abdel Aziz al-Hakim, took over from his brother, Muhammad Baqir al-Hakim, after he was assassinated in a bombing in 2003 - which some blame on Zarqawi's Tawhid and Jihad group.
Sciri is one of the largest parties in the Shia coalition which dominates the Iraqi parliament, and still has strong Iranian connections.
In 2005, under Sciri minister Bayan Jaber the Interior Ministry was accused of recruiting large numbers of the militia's members into the security forces and turning a blind eye to death squads targeting Sunnis.
Mr Jaber, a former Badr Brigade commander, denied any Interior Ministry involvement in sectarian killings.
In January 2006, four policemen arrested by US forces after saying they were going to execute a Sunni man were found to be linked to the Badr Brigade.
But there has been little other evidence to substantiate the widely-made accusations against the organisation.
Both the Badr Brigade and Mehdi army have strong presences in the southern city of Basra.