How bad is it?
It is hard to calculate reliable figures for the dead and wounded because of the chaotic state of Iraq's institutions.
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An estimate by US scientists in October 2006 suggested that about 655,000 civilians had been killed since the 2003 US-led invasion.
Western human rights campaigners give much lower figures - between 42,000 and 47,000 civilians - but point out many deaths probably go unreported.
About 3,000 coalition troops - more than 2,800 of them Americans - have died in Iraq in the same period.
The monthly Iraqi death toll hit a record high in October 2006, with more than 3,700 people losing their lives, according to a UN report.
Why is it so intense?
Insurgents, some loyal to the former regime and others pursuing jihad against the Western forces, are conducting a campaign of violence aimed at driving out US-led foreign forces.
Another aim of some elements of the insurgency is to bring about a sectarian civil war and some analysts say they have succeeded in this.
Some of the deadliest violence has been seen between Iraq's two main Arab communities, the Shia and Sunni Muslims.
Iraq's other communities, Kurds and Christians, have also suffered and violent ordinary crime, such as kidnapping for ransom, is rife.
Fighting involving coalition and Iraqi government forces on the one hand, and the armed groups on the other, has also taken a heavy civilian toll.
Who are the armed groups?
The insurgents can be divided into a number of different groups but they are mostly Sunni Muslims.
Al-Qaeda in Iraq is the best-known of the jihadi groups and has been blamed for many of the bombings and beheadings.
The Mehdi Army is the best-known of the Shia Iraqi militias, which act like a police force in many areas and have been accused of many sectarian attacks.
Some members of the Iraqi army and police force have allegiances to groups involved in the violence.
What is the political cost?
Iraq's government says there can be no reconciliation and economic recovery until the violence stops.
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But its forces remain weak.
Western leaders say they do not want to keep their troops in Iraq indefinitely but fear that pulling them out would embolden the militants and make things even worse.
And many fear that the conflict is radicalising Muslims in the rest of the world.