A sweeping, multi-year inquiry into Britain’s role in the Iraq War delivered a devastating assessment Wednesday, with investigators blaming the country’s political, military and intelligence leadership for disastrously mismanaging a conflict that need never have started and that ended “a very long way from success.”

The findings offer official validation to the views of the Iraq War’s most ardent critics, forensically eviscerating in the sober language of the British civil service nearly every aspect of the conflict’s conception, planning and execution.

In a country where the shadow of Iraq continues to loom over both politics and policy, the ­report’s documentation of a disaster in the making guided by American allies could shape British ­decision-making for years or even decades to come.

It could also permanently taint the legacy of the country’s prime minister at the time, Tony Blair, who came in for the sharpest critique of all — and responded with a marathon news conference on Wednesday afternoon at which he was by turns deeply contrite and pugnaciously defiant.

“I express more sorrow, regret and apology than you may ever know or can believe,” Blair said, his voice cracking with emotion.

But for all his misgivings over what he acknowledged were failures of planning and preparation, he insisted that the war was still worth it because it rid the world of the dictator Saddam Hussein.

“I did it because I thought it was right,” said a graven-faced Blair, the country’s prime minister for a decade.

The findings by a team of British investigators are the culmination of seven years of work in which they were given nearly unfettered access to British documents and witnesses. Delivered in a breathtaking 2.6 million words — five times the length of “War and Peace” — the focus was on Britain’s role as both invader of Iraq in 2003 and an occupier for the next six years.

But the report inevitably also cast light on U.S. decision-making. The United States, which led the march to war, has never conducted a comparably ambitious study of its own failings in Iraq.

The report includes memos that Blair sent to then-President George W. Bush — though not the replies. In one such memo, from July 2002, Blair writes to Bush that “I will be with you, whatever” — suggesting a British blank check for war at a time when other European allies were strongly opposed.

Former British prime minister Tony Blair reacts to the sweeping Chilcot report of Britain's role in the Iraq War, which blamed the country’s political, military and intelligence leadership for a conflict that could have been avoided and that ended “a very long way from success.” (AP)

With violence in Iraq still raging — a bombing in Baghdad on Sunday left more than 250 people dead — the inquiry casts blame widely for a conflict that cost the lives of 179 British troops and, at the time of the British withdrawal in 2009, at least 150,000 Iraqis. To date, more than 4,500 Americans have died in Iraq and more than 32,000 have been wounded.

The war was initially sold to the public on both sides of the Atlantic as a vital intervention to deprive Hussein of weapons of mass destruction. But no such weapons were ever found.

With exacting detail, the report catalogues a succession of failures.

British intelligence painted a flawed picture of Iraqi military capacity, with agencies never doubting the existence of WMDs. In fact, the report concluded, Iraq posed “no imminent threat” to Britain. In making their case to the public, Blair and other British officials described the case against Hussein “with a certainty that was not justified.”

In their private deliberations, they ignored warnings that the invasion of Iraq could be a boon to Islamist extremists. Groups such as al-Qaeda gained key footholds amid Iraq’s chaos, and militant offshoots later became the foundation for the Islamic State.

The British relied almost exclusively on their American counterparts for postwar planning, then failed to deliver the manpower and resources needed to make good on promises to transform Iraq into a functioning, stable democracy.

In a statement delivered Wednesday in London, the report’s lead author, retired civil servant John Chilcot, said Blair took the country to war “before the peaceful options for disarmament had been exhausted. Military action at that time was not a last resort.”

Chilcot, speaking at a conference center in central London, was applauded by relatives of British troops who were killed in the war. Wiping away tears, family members later praised the inquiry and said it had opened a pathway to possible legal action against Blair and other British officials responsible for launching the war.

Sarah O’Connor, whose brother, Sgt. Bob O’Connor, was killed in 2005, described Blair as “the world’s worst terrorist.” Roger Bacon, whose son also died in 2005, said the report should be used “to ensure that all aspects of the Iraq war fiasco are never repeated again.”

Outside, protesters held aloft signs reading “Bliar” and chanted, “Blair lied, thousands died!”

The report was commissioned by Blair’s successor, Gordon Brown, and was originally expected to take a year to complete.

Instead, it took seven years amid persistent delays that Chilcot has said reflected his own underestimation of the scale of the challenge involved with assessing Britain’s first invasion and full-scale occupation of a sovereign state since World War II.

The report will give ample ammunition to the war’s toughest critics, including those in Britain who have called for war crimes charges to be brought against Blair — who now runs a lucrative consulting business that has drawn scrutiny for working with authoritarian governments in the Middle East and elsewhere.

The leader of Britain’s Labour Party, Jeremy Corbyn, on Wednesday described the war as “illegal” and apologized on behalf of Blair’s party to “the millions of British citizens who feel our democracy was traduced and undermined.”

Others in Parliament floated the idea of formally censuring Blair. But the report takes a pass on the issue of Blair’s legal culpability, and he is unlikely to be charged.

Chilcot said the question was beyond the scope of his inquiry and could “only be resolved by a properly constituted and internationally recognized court.”

In two hours of passionate statements after the report’s release, Blair said that “not a single day” passes without him thinking over the Iraq War decisions.

Yet he stood firm on the question of whether he had deceived the public, saying he had taken the country to war “in good faith,” and that the report had validated his contention that “there were no lies” from his government.

That view was echoed by the man who now holds Blair’s job, David Cameron, who said he did not believe the report had found evidence of “deliberate deception.”

Speaking in the House of Commons, Cameron urged politicians to learn the lessons of the inquiry, the first being that “taking the country to war should always be a last resort.”

Wednesday’s report lands as Britain continues to reckon with the aftermath of its June 23 vote to exit the European Union, an outcome that prompted Cameron’s resignation and that has spawned a mutiny against Corbyn from within his own ranks.

The report does not have a direct bearing on the country’s current political chaos, but it is likely to revive for many Britons memories of a rush to war that has come to epitomize betrayal by the nation’s elites. The cynicism of British voters that the Iraq War helped to spawn was on display last month, when many seemed to blithely ignore the warnings of experts that a British exit from the E.U. could spark economic and political chaos.

The shadow of the Iraq War also influenced Britain’s 2013 decision to stay out of a U.S. plan to launch airstrikes against the government of Syrian President Bashar al-Assad, and to delay joining in when the United States began bombing Islamic State targets a year later.

Even as Wednesday’s report ignited a reckoning in Britain on Wednesday, the reaction in Iraq was relatively muted among people too focused on daily survival to worry about another report documenting the West’s failures in their country. After 13 years of violence, the war to depose Hussein hardly seems worth it even to those who celebrated his fall.

Bush and Blair “are war criminals and should be punished,” said Kadhim Sharif Hassan, 60, who famously took a sledgehammer to a statue of Hussein in central Baghdad in 2003 and now says it would have been better if the dictator had not been overthrown.

“The system they created is much worse than the old system. It’s only produced death and destruction,” he said.

Karla Adam in London, Brian Murphy in Washington, Loveday Morris in Beirut and Mustafa Salim in Baghdad contributed to this report.