The U.S.-led invasion of Iraq in 2003 ended nearly a quarter of a century of authoritarian rule under Saddam Hussein, but his removal hardly brought the peaceful transition to democracy envisaged by Washington.

Tens of thousands of Iraqis have died since the invasion. Many have been killed in sectarian violence which erupted in early 2006.

Following the invasion a Sunni-led insurgency attacked U.S.-led troops, Iraq's Shi'ite dominated government, its fledgling security forces, oil installations and civilians - many of them the victims of devastating suicide and car bombings.

The insurgents included nationalists, former members of the Iraqi military and supporters of Saddam. Al Qaeda militants made up around 5 percent, according to U.S. officials and Sunni politicians.

Sunnis accused Shi'ite militia of carrying out revenge killings.

A 2007 report from British think tank Chatham House said there was not one civil war in Iraq, but many civil wars and insurgencies involving a number of communities and organisations struggling for power.

The violence drove many people from their homes. Iraqis became increasingly reliant on food aid and healthcare deteriorated in many parts of the country.

Aid workers were kidnapped and killed. Many international humanitarian organisations pulled foreign staff out of the country or left altogether.

The dire security has slowed reconstruction and attacks on the country's oil infrastructure have cost billions of dollars in lost revenue.

But the bloodshed appears to have peaked with violence dropping significantly since mid-2007.

This has been attributed to a major U.S. military build-up, a more aggressive strategy towards al Qaeda and Shi'ite militias, and Shi'ite cleric Moqtada al-Sadr's order to his Mehdi Army militia to freeze their activities.

Others say violence has declined because areas have been ethnically cleansed during two years of sectarian bloodshed.

At the end of 2007, Britain handed security for Basra to Iraqi forces, effectively ending nearly five years of British control of southern Iraq. By November 2008 the U.S. military had handed over security responsibility for 13 out of Iraq's 18 provinces.

Provincial elections in January 2009 were relatively peaceful - a marked contrast from parliamentary elections in January 2005 which helped put Iraq on the path to civil war.

U.S. and British forces invaded Iraq in March 2003, accusing Saddam of failing to cooperate with U.N. inspections searching for alleged weapons of mass destruction. The invasion was opposed by France, Germany, China and Russia among others.

Saddam was overthrown on April 9 after U.S. troops took Baghdad. His ruling Baath party and the Iraqi army were disbanded, leaving hundreds of thousands of men jobless.

Inspectors have since concluded that pre-war Iraq did not have stockpiles of biological and chemical weapons and that its nuclear programme had already decayed.

Following his capture, Saddam, who came from the Sunni minority, was put on trial for the killing of 148 Shi'ites in 1982. He was found guilty and executed at the end of 2006. He was also charged with genocide against Kurds in the late 1980s, but was executed before the trial ended. Kurdish authorities say tens of thousands were killed and hundreds of thousands displaced in a campaign to assert government control over Kurdish areas in the north.

Although Iraq is predominantly Shi'ite - about 60 percent - the country has traditionally been ruled by the Sunni minority, who form about 20 percent of Iraqis. The rest of the population is mostly made up of Kurds. Smaller minorities include Turkmen and Assyrians.

Saddam had already been the country's effective strongman for a decade when he came to power in 1979.

Iraq and Iran went to war the following year, relations having deteriorated after the 1979 Islamic revolution in Iran. The eight-year conflict left up to 1 million dead and Iraq heavily indebted. The war helped Saddam justify the creation of a nuclear programme and the world's fourth largest army.

Saddam used chemical weapons on Iranian troops and later against Iraqi civilians, most notably in an attack on the town of Halabja in 1988 which killed 5,000 Kurds, according to human rights activists.

Iraq's repression of the Kurds and Shi'ites did not attract huge international attention at the time. But the 1990 invasion of tiny Kuwait to the south was not overlooked. A U.S.-led force expelled the Iraqis from the oil-rich nation the following year.

The invasion triggered international sanctions. The United States, United Kingdom and France imposed no-fly zones over the Kurdish north in 1991 and the Shi'ite south in 1992 to protect the local populations. France later pulled out of patrolling the zones.

In an attempt to minimise the impact of sanctions on ordinary Iraqis, the United Nations administered an oil-for-food programme from 1996 onwards, which allowed the regime to sell oil in return for humanitarian goods.

This provided some relief but also appears to have enriched Saddam and those around him. An independent inquiry after the U.S.-led invasion found thousands of companies had colluded with Saddam's government to skim $1.8 billion off the programme.

No one knows how many civilians have died since the invasion. The campaign group Iraq Body Count tallies the numbers reported in the media.

In October 2006, U.S. and Iraqi researchers said as many as 600,000 Iraqis had died in violence - although their data did not distinguish between civilians and combatants. And they said another 54,000 had died beyond the norm, prevented by violence from getting healthcare, for example. These numbers were dismissed by the Iraqi, U.S. and British governments.

Iraqi government and World Health Organisation researchers have estimated the civilian death toll for the same period at around 150,000.

But British polling group Opinion Research Business said in 2008 that their research showed more than a million people had died as a result of the war. Their survey of 2,414 adults found 20 percent of people had had at least one death in their household.

The Sunni-led insurgency attacked foreign troops and anyone deemed to support them. Bombers killed Iraqi men at recruitment posts and training centres for the new police force and army. They have also blown up mosques, markets and other civilian areas.

Humanitarian workers have been another target.

• In August 2003 a truck bomb exploded at the U.N. headquarters in Baghdad, killing 22 people, including envoy Sergio Vieira de Mello. The organisation subsequently withdrew all non-Iraqi staff.

• Twelve people were killed when a car bomb hit the International Committee of the Red Cross in October 2003, prompting the organisation to pull out foreign staff.

• Many foreign aid agencies began withdrawing following the abduction of two Italian aid workers in September 2004. The hostages were released but World Vision's Iraqi head of operations was killed the same month. The Paris-based aid group Action Contre la Faim said aid activities by U.S.-led forces made it hard for charities to appear impartial.

• Militants kidnapped and killed British aid worker Margaret Hassan, who ran CARE International's operations in Iraq, in late 2004. Her death shocked many because she had lived in Iraq for decades and opposed the invasion. An Italian aid worker Salvatore Santoro was kidnapped and killed at the end of 2004.

More than 200 foreigners and thousands of Iraqis have been abducted since the invasion. At least 61 foreign hostages have been reported executed.

Sectarian violence spiralled in early 2006 following the bombing of the Golden Mosque in Samarra, one of the holiest Shi'ite shrines.

Many Iraqis fled their homes as death squads sought to clear neighbourhoods of those they saw as outsiders. Victims often showed signs of torture.

Shi'ite militia leaders said they were responding to Sunni-led violence. Sunnis said the militia had links with the Shi'ite dominated police and accused them of killing thousands.

An estimated 2.8 million people were internally displaced as of July 2008, according to the Internal Displacement Monitoring Centre (IDMC). Another 2.3 million people have left the country, mainly for Syria and Jordan, the U.N. refugee agency (UNHCR) says.

But it should be pointed out that a significant number of these people had fled their homes before the 2003 invasion. In 2002, more than 700,000 Iraqis were internally displaced and 420,000 were refugees.

Host governments are struggling to meet the needs of Iraqi refugees. Syria - which has taken in about 1.4 million Iraqis - imposed tighter entry restrictions in October 2007, effectively closing its border to any more refugees.

In response, the UNHCR called for the introduction of a "humanitarian visa" for Iraqis.

Around 870,000 internally displaced Iraqis have sought safety in the more stable north of the country.

The predominantly Kurdish region has remained relatively peaceful. But neighbouring Turkey says Kurdish rebels are using northern Iraq as a base for attacks on Turkish soil.

Turkey began amassing troops along the border in late 2007 and has launched air attacks and cross-raids.

Aid agencies have warned that military conflict could displace thousands more Iraqis.

Iraqis once enjoyed some of the highest living standards in the region. But poverty grew under Saddam, and the 2003 invasion triggered a humanitarian crisis beyond the scale that aid agencies predicted in the run-up to the war.

The humanitarian situation in Iraq is among the most critical in the world, according to the International Committee of the Red Cross. Millions of Iraqis lack clean water and proper sanitation and the health care system is "now in worse shape than ever", the ICRC says in a 2008 report.

The poor public water supply has forced some families to use at least a third of their average $150 monthly income buying clean drinking water, it says.

The shortage of drinking water has contributed to several outbreaks of cholera.

According to a survey published by the World Food Programme in November 2008, nearly a million people need emergency food aid - a drop from 4 million in 2005. The agency warns, however, that another 6.4 million are highly dependent on safety nets such as the government-run Public Distribution System. This is theoretically available to all Iraqis but, when people are displaced, they are frequently unable to register to receive the monthly rations.

Children - who make up almost half Iraq's population - are particularly at risk. Child malnutrition rates rose from 19 percent before the invasion to 28 percent in 2007, according to Oxfam.

Many Iraqis who have fled their homes do not have proper access to health care.

The instability has compromised aid agencies' ability to work in the country. Many have scaled back their presence or pulled out. Since 2003, at least 94 aid workers have been killed, 248 injured, 24 arrested or detained and 89 kidnapped or abducted, according to the NGO Coordination Committee in Iraq.

The United Nations, which withdrew most of its 600 staff from Baghdad in 2003, is now expanding its role in Iraq following a U.N. Security Council resolution.

The agency will have a powerful advisory role in Iraqi affairs, allowing it to help the government settle political, economic and constitutional matters, internal border disputes and to foster dialogue between the country's different factions.

A 2003 international donor conference generated pledges of more than $32 billion in loans and grants for the period 2004 to 2007. The United States, the largest donor, promised $18.4 billion.

But reconstruction has lagged, even though Iraq sits on some of the world's largest oil reserves.

The violence is partly to blame. Early on looters stripped newly rebuilt facilities, while attacks on westerners or anybody working with foreign companies slowed work to a crawl. The cost of keeping workers safe has soared.

Another major problem threatening reconstruction is corruption.

"If urgent steps are not taken, Iraq will not become the shining beacon of democracy envisioned by the Bush administration, it will become the biggest corruption scandal in history," global watchdog Transparency International has warned.

Iraq was ranked third worst in Transparency International's 2008 survey of perceived levels of public sector corruption in 180 countries.

A report by the U.S. special inspector general for Iraqi reconstruction says corruption in the oil and gas sector could have "devastating effects" on efforts to stabilise the country.

Sunnis have accused Shi'ite militia of involvement in smuggling oil.

Iraq was producing about 2.3 million barrels per day in 2008.

U.S. officials originally predicted Iraq's oil would fund relief and reconstruction projects. But efforts to rejuvenate the oil sector have suffered setbacks, including insurgent attacks on energy facilities.

Meanwhile, lack of agreement on how to divide the oil revenue among Shi'ite, Sunni and Kurdish communities is one of the main obstacles for reconciliation.

Although the United States formally returned sovereignty to an Iraqi interim government in June 2004, it still has about 140,000 troops in the country, down from a 2007 peak of 160,000-170,000.

In its campaign to stamp out the insurgency the U.S. military has been accused of killing civilians, mass detentions and torture.

U.S. troops have said they were acting in self defence in some cases, but the military has launched investigations into several incidents.

In one of the worst cases soldiers gang raped and murdered an Iraqi teenager and shot dead her parents and younger sister in March 2006. Three of the soldiers involved received jail sentences between 90 and 110 years. The crime significantly increased tensions with U.S. forces in Iraq.

Allegations of prisoner abuse have also dogged the U.S.-led forces. In 2004 photographs emerged of Iraqi detainees being sexually humiliated, threatened with snarling dogs and otherwise mistreated in Abu Ghraib prison on the outskirts of Baghdad.

The U.S. government says it has put on trial scores of armed forces staff accused of prisoner abuse in Iraq and Afghanistan.

Under a security pact which came into force on January 1, 2009, U.S. forces must withdraw from Iraqi towns by mid-2009 and leave Iraq altogether by the end of 2011. The United States also lost the power to hold prisoners indefinitely without charge - at the time of the pact it held about 17,000 detainees in Iraq. And U.S. contractors became subject to Iraqi law, able to be prosecuted in Iraqi courts.