Tony Blair would like to be remembered for transforming Britain's schools and bringing peace to Northern Ireland, but for many his premiership was defined by the Iraq War.
He was Labour's most successful leader, winning his party a record three elections that began with the historic 1997 landslide.
His time as prime minister saw the UK enjoy a record period of economic growth, low unemployment and large increases in spending on health and education.
But all this was overshadowed by his decision to lend British support to US president George Bush's war on Iraq in 2003 despite massive public opposition.
As a successful invasion turned into a disastrous occupation that cost the lives of 179 UK personnel, Mr Blair, 56, found himself the target of grieving military families' anger.
There were calls for him to be indicted for war crimes, and one father whose son was killed in Iraq even stood against him at the 2005 general election.
The reasons why Mr Blair decided to take Britain to war have been widely debated, and form a key part of the Chilcot inquiry into the conflict.
In his important "Chicago speech" in April 1999, he set out five tests for deciding "the most pressing foreign policy problem we face", namely when it was right to intervene in other people's conflicts.
Mr Blair demonstrated his belief that taking military action against oppressive regimes could be justified with his strong support for the Nato bombing campaign against Yugoslavia in 1999.
The September 11 2001 terrorist attacks on the United States raised the stakes dramatically as Washington turned its attention to its enemies around the world.
Mr Blair supported Mr Bush's invasion of Afghanistan and - according to former Number 10 communications director Alastair Campbell - assured him Britain would "be there" if it came to military action in Iraq.
America had been officially committed to the overthrow of Saddam Hussein since 1998, but the UK was more cautious about regime change as an aim in its own right.
Britain therefore concentrated on the weapons of mass destruction (WMD) that Iraq was thought to have stockpiled, arguing that they posed a serious threat to international security.
Mr Blair encouraged Mr Bush to ramp up pressure on Saddam by seeking a new United Nations Security Council resolution on his weapons programmes.
This was achieved with Resolution 1441 in November 2002, which led to weapons inspectors returning to Iraq.
No WMD were found, but London and Washington were not satisfied that Saddam was complying with the inspection regime and moved rapidly to war.
Despite widespread unrest within the Labour Party and an anti-war march in London in February that attracted up to two million protesters, Mr Blair comfortably won a Commons vote on military action days before the start of the conflict.
The US and the UK supplied the vast majority of troops for the invasion of Iraq, which began on March 20 2003.
Coalition forces quickly captured the major cities and were widely greeted by cheering Iraqis overjoyed to be free of Saddam's brutal yoke.
But there had been little post-invasion planning and the country soon descended into chaos as old regime loyalists and Islamic militants launched a bitter insurgency.
Suicide bombings aimed at foreign troops and civilians alike became a daily nightmare, turning Iraq into a turbulent bloodbath.
Mr Blair faced growing pressure as the British death toll rose and it became increasingly clear that no WMD were going to be discovered.
He was visibly shaken by the July 2003 suicide of Government weapons expert Dr David Kelly, the source of a BBC report that an official dossier about Saddam's weapons was "sexed up".
The Hutton inquiry into the incident cleared the Government but was widely seen as a whitewash, and the subsequent Butler inquiry into the quality of the intelligence which led to the war was damning.
Even now, nearly seven years after the invasion and more than two and a half years after he left office, Mr Blair is still haunted by Iraq.
In October he was confronted by the parents of two servicemen killed in the war at a reception after a memorial service for the British personnel who died in the conflict.
And he told TV presenter Fern Britton in a BBC interview in December that he believed it would still have been right to invade Iraq even if it was known at the time that Saddam did not have WMD.
The former prime minister may be hoping to draw a line under the war when he returns to Westminster today to address the Chilcot inquiry, but this will almost certainly be in vain.