Thirteen years after British troops marched into Iraq and seven years after they left a country that's still mired in violence, John Chilcot's long-delayed, 2.6 million-word report on the divisive war and its chaotic aftermath addresses the lingering question: What went wrong?

Chilcot's inquiry held public hearings between 2009 and 2011, taking evidence from more than 150 witnesses — including former British prime minister Tony Blair. The inquiry analysed 150,000 documents and cost more than 10 million pounds ($13 million), but its report has been repeatedly delayed, in part by wrangling over the inclusion of classified material — including conversations between Blair and former US president George Bush.

Opponents of the war claim Blair's government exaggerated evidence that Iraqi dictator Saddam Hussein had weapons of mass destruction that threatened the West — the foundation of the case for war. No chemical, biological or nuclear weapons were found in Iraq. Previous inquiries, limited in scope, had largely absolved the government of blame.

In this picture gallery, IBTimes UK looks back at Britain's involvement in the Iraq war between 2003 and 2009.

Iraq war
14 March 2003: British troops gather for a briefing by US Lieutenant General Jeff Conway Commanding General 1st Marine Expeditionary Force in the Kuwait desertDan Chung/Reuters
Iraq war
16 March 2003: Former British Prime Minister Tony Blair stands with former Spanish Prime Minister Jose Maria Aznar and former US President George W Bush during a press conference at the Lajes Field Airbase in TerceiraKieran Doherty/Reuters
Iraq war
20 March 2003: British armoured vehicles drive past burning oil wells in southern IraqBruce Adams/Reuters
Iraq war
21 March 2003: Smoke billows from a presidential palace compound in Baghdad during air strikes, as US and British ground forces advancing across southern Iraq battled for hours for control of a strategic airfieldGoran Tomasevic/Reuters
Iraq war
21 March 2003: RAF Group Captain Al Lockwood briefs the media about the crash of a US helicopter in Kuwait. Eight British Royal Marines from 3 Commando Brigade and four American servicemen perished in the accidentCarlo Allegri/Getty Images
Iraq war
22 March 2003: A British soldier gives the 'thumbs up' as a convoy of armoured vehicles rolls towards BasraReuters
Iraq war
24 March 2003: Soldiers from the 7th armoured brigade, the Desert Rats, pose with a picture of then Iraqi president Saddam Hussein taken from the headquarters of the local Ba'ath party on the outskirts of BasraMark Richards/Reuters
Iraq war
24 March 2003: A British armoured combat vehicle knocks down a picture of Saddam Hussein in the city of BasraMark Richards/Reuters
Iraq war
25 March 2003; British Royal Marines Omar Rawlings and Del Morton of 40 Commando smile as they squeeze into their dug-in position in mud after heavy rainstorms north of the Al Faw peninsula on the approach to BasraTerry Richards/Reuters
Iraq war
27 March 2003: Iraqi children welcome British troops from 42 Commando as they go on patrol in the town of Um Qa-eel, south of BasraJon Mills/Reuters
Iraq war
5 April 2003: British forces fire on Iraqi targets from a position near the city of BasraDan Chung/Reuters
Iraq war
7 April 2003: British soldiers are surrounded by Iraqi children in front of a mural of former Iraqi dictator Saddam Hussein at Siyamar Square in the old city of Basra. About 400 British and US troops, guarded by tanks and helicopter gunships, walked unopposed almost to the centre of Iraq's second cityYannis Behrakis/Reuters
Iraq war
7 April 2003: Royal Marine Stuart Lawley, 20, from Darlington stands in a corridor of Saddam Hussein's palace in BasraJon Mills/Reuters
Iraq war
2 May 2003: A British soldier stands on top of a terminal at Basra International Airport ahead of the landing of a 747 carrying some 60 tonnes of medical supplies donated by Virgin Atlantic airlinesGraeme Robertson/Getty Images
Iraq war
29 May 2003: Prime Minister Tony Blair addresses British troops in Basra, becoming the first Western leader to visit Iraq since the war that toppled Saddam HusseinStefan Rousseau
Iraq war
17 July 2003: Former Prime Minister Tony Blair gives a thumbs up after addressing a joint meeting of Congress at the US Capitol in Washington. He said that history would forgive America and Britain for invading Iraq even if they were proved wrong about the threat of lethal weaponsJason Reed/Reuters
Iraq war
15 July 2003: Dr David Kelly arrives to give evidence to the Commons foreign affairs select committee after he was named by the government as the possible source of a BBC report claiming Downing Street "sexed up" an Iraq WMD dossierStephen Hird/Reuters
Iraq war
18 July 2003: An aerial view shows British police officers at Harrowdown Hill where the body of British government scientist Dr David Kelly was found near his home in OxfordshireStephen Hird/Reuters
Iraq war
14 December 2003: Television screens show images of former Iraqi dictator Saddam Hussein in the control room of ITN (Independent Television News) in London. Saddam was captured by US forces during a raid near his home town of TikritIan Waldie/Getty Images
Iraq war
22 March 2004: British Army troops are covered in flames from a petrol bomb attack during a violent protest in BasraAtef Hassan/Reuters
Iraq war
24 March 2004: A British soldier secures an area near a burning oil pipeline near BasraJoseph Barrak/AFP
Iraq war
2 July 2004: Soldiers from the The Royal Welch Fusiliers prepare to board a Chinook helicopter as they carry out patrols around Basra following several attacks on oil pipelines and facilities in the regionGiles Penfound/British Army via Getty Images
Iraq war
28 July 2004: Daoud Mousa, whose son Baha Moussa who died while in the custody of British soldiers in Iraq, shows photographs of his son and family to the press as he arrives at the High Court hearing on human rights violationsGraeme Robertson/Getty Images
Iraq war
10 October 2004: British hostage Ken Bigley is pictured in a video, released by the terrorist group Tawhid wa Jihad, pleading for his life moments before being brutally murderedGetty Images
Iraq war
28 November 2004: An armoured vehicle from the Black Watch regiment manouevres in the sand in the British forward base of Al Quaqa near the city of YusufiyahOdd Andersen/AFP
Iraq war
Photograph number 20, showing a soldier "simulating" kicking an Iraqi detainee (wet patches are water) used in the Court Martial in Osnabruck, Germany, where three soldiers from the Royal Regiment of Fusiliers were facing a court martial over the ill-treatment of civilian prisoners. In February 2005, Cpl Daniel Kenyon was sentenced to 18 months in prison, L/Cpl Mark Cooley to two years and L/Cpl Corporal Darren Larkin to 140 daysPA
Iraq war
2 June 2005: Late Brian Haw stands outside the Houses of Parliament where he lived and slept for years, protesting against UN sanctions, the invasion and occupation of IraqCarl de Souza/AFP
Iraq war
19 September 2005: A British soldier jumps from a burning armoured vehicle that was set ablaze by protesters after a shooting incident in BasraAtef Hassan/Reuters
Iraq war
19 September 2005: A young Iraqi protester lobs stones at British soldiers during clashes in BasraEssam al-Sudani/AFP
Iraq war
6 October 2005: Former British Prime Minister Tony Blair and ex Iraqi President Jalal Talabani hold a joint news conference at Downing StreetAdrian Dennis/AFP
Iraq war
18 March 2006: On the third anniversary of the start of the war, protesters march through London to call for withdrawal of British troops from IraqScott Barbour/Getty Images
Iraq war
25 May 2006: Former US President George W Bush and ex British Prime Minister Tony Blair leave the East Room of the White House after speaking to reporters about IraqKevin Lamarque/Reuters
Iraq war
16 October 2006: A boy throws a rock at a burning British consulate vehicle after it was hit in a rocket propelled grenade attack in BasraAtef Hassan/Reuters
Iraq war
17 December 2006: Former Prime Minister Tony Blair puts his signature on an armoured carrier during a  visit to British troops in BasraEddie Keogh/Reuters
Iraq war
3 September 2007: British armoured vehicles arrive at a contingency operating base after pulling out from Basra PalaceBritish Forces Media Office/Reuters
Iraq war
17 December 2008: Former Prime Minister Gordon Brown addresses British troops at Basra Air Station during his one-day visit. He confirmed that British troops would leave Iraq the following year after "building a democracy for the future"Peter Macdiarmid/Getty Images
Iraq war
10 February 2009: A soldier enters the Subway fast food oulet at the main British base at Basra AirportMatt Cardy/Getty Images
Iraq war
27 May 2009: Corporal Chris Smyth of 15 Squadron Royal Air Force, the last British combat forces to leave Basra, hugs his five-month-old son Ethan at RAF Honington in SuffolkStephen Hird/Reuters
Iraq war
15 June 2009: Demonstrators from the Stop the War Coalition protest outside the Houses of ParliamentStephen Hird/Reuters
Iraq war
9 October 2009: Former Prime Minister Tony Blair and his wife Cherie leave a Service of Commemoration to mark the end of combat operations in Iraq, at St Paul's Cathedral in LondonKieran Doherty/Reuters
Iraq war
179 British military personnel lost their lives during the Iraq warMoD

The US-led conflict killed 179 British troops and some 4,500 American military personnel. It also helped trigger violence that killed hundreds of thousands of Iraqis and still rocks the Middle East. Iraq descended into sectarian strife after the occupiers dismantled Saddam Hussein's government and military apparatus, unleashing chaos that helped give rise to the Sunni extremist militants of the Islamic State group.