The Obama administration is bracing for the imminent disclosure by the WikiLeaks website of a vast cache of secret U.S. Iraq war documents, which could throw a light on some of the darkest episodes of that conflict.

WikiLeaks, a self-described whistle-blower website, is expected to post up to 400,000 documents online this week after having shared them in advance with several news organizations.

It would be the second major release of classified U.S. war reports by WikiLeaks in the past four months. In July, despite objections by the U.S. government, the international anti-secrecy group posted nearly 77,000 documents from the Afghan conflict on its website.

Together, the two sets of disclosures would represent a massive breach of U.S. information security and raise questions about the viability of post-9/11 government policies that expanded the distribution of classified information as a means of improving coordination among intelligence and security agencies.

Military officials believe the Iraq documents will emphasize the failings of the Iraqi government and military, including allegations of Iraqi mistreatment of detainees and ineptitude in combat. They also are likely to include some classified U.S. diplomatic cables, officials said.

The Afghan documents released in July gave a ground-level view of that war from 2004 through 2009, based largely on raw U.S. military intelligence reports, including sensitive material that revealed names of Afghan informants and provided details of Afghan civilian casualties.

In the aftermath, the Pentagon pressed its investigation of the security breach, and the Defense Intelligence Agency set up a 120-person Information Review Task Force to analyze the leaked documents and their impact on the war effort. Congress demanded an assessment of the damage to U.S. national security.

Pentagon officials said last week that no significant U.S. intelligence sources or practices were compromised by the leak of Afghan war reports. But Defense Secretary Robert Gates wrote in a letter to Congress in August that revealing names of Afghans who cooperated with U.S. forces was "likely to cause significant harm or damage" to U.S. national security interests.

In anticipation of the release of some 400,000 secret files on Iraq that Pentagon officials believe someone slipped to WikiLeaks, the DIA group has combed through them in search of records containing especially sensitive information such as intelligence sources and criticism of Iraqi culture or Islam.

Based on that review, the U.S. Central Command already has been notified of the names of Iraqis and allies mentioned in the records, as well as other information that could put those working with the U.S.-led multinational force in danger.

Following July's document release, WikiLeaks editor-in-chief Julian Assange was asked by some human rights groups to remove civilian names from the report, and the Pentagon publicly called on him to return the documents. Assange also became the target of allegations of sexual misconduct in Sweden.

The latest WikiLeaks revelations may not change public perceptions of the Iraq war. But they could provide new details about a conflict that seemed headed for a quick victory after the fall of Baghdad in April 2003, only to turn into a protracted, deadly counterinsurgency struggle.

A number of U.S. mistakes in the conflict have already been documented.

Invading U.S. forces found no weapons of mass destruction, despite expectations stoked by President George W. Bush and his defense secretary, Donald H. Rumsfeld. Pictures from Abu Ghraib prison of grinning U.S. soldiers posing with naked men on leashes helped recruit insurgents.

But some unanswered questions remain.

The WikiLeaks documents could shine light on the roots of the insurgency and its expansion. Was it part of the U.S. failure to understand Iraq's sectarian divisions and the depth of its economic decay? Or were Iraqis driven to fight by the heavy-handed use of U.S. military power?

The documents may show how U.S. officials struggled to grasp the size, strength, motivations and leadership structure of the insurgency.

They could also provide new details of the manhunt for Abu Musab al-Zarqawi, the leader of the shadowy group known as al-Qaida in Iraq, by a highly secret U.S. special operation unit. The Jordanian was finally cornered in house northeast of Baghdad in June 2006 and killed by U.S. bombs.

Another key moment in the war — the documents could shed light on it, too — came on March 31, 2004, when mobs in Fallujah ambushed U.S. security contractors and hanged their charred and mutilated remains on a bridge over the Euphrates.

The incident ultimately led to the November 2004 Marine-led assault on Fallujah, which finally recaptured a city that had been an al-Qaida stronghold.

At the center of the WikiLeaks controversy is a former intelligence analyst, Army Pfc. Bradley Manning, who is under suspicion of having provided the classified military documents to WikiLeaks.

Manning was stationed in Iraq when he was arrested by U.S. authorities. He is now charged with multiple counts of mishandling classified data and putting national security at risk.

He stands accused of accessing more than 150,000 classified State Department cables, and is a person of interest in the investigation of the July 25 WikiLeaks release. That investigation is headed by the Army's Criminal Investigation Command.

WikiLeaks describes itself as a public service organization whose mission is to "protect whistle-blowers, journalists and activists who have sensitive materials to communicate to the public."