Amid the fallout from the Brexit decision and the internecine battles for leadership in both the Conservative and Labour parties, it would be easy to underplay The Chilcot Inquiry, or to wonder why it is necessary. Don't be fooled, though, it may be more necessary now than ever. Chilcot said this morning, "We have concluded that the UK chose to join the invasion of Iraq before the peaceful options for disarmament had been exhausted," a damning statement.

Though it is the third official British Inquiry into the events surrounding the Iraq War of 2003, it is the first which has been given the remit to properly investigate how the UK became embroiled in one of the most calamitous foreign policy decisions of the last 70 years.

The Suez Crisis is seen as the benchmark for disastrous foreign interventions since World War Two. Anthony Eden's hubristic decision to ally with France and Israel to invade Suez and ignore American advice ended in humiliation. Militarily, the operation could have been a success, but the impact it had on Britain's standing in the world was severe. 

Less than 10 years later, Harold Wilson was Prime Minister of the UK. With American forces swarming into Vietnam under Lyndon Johnson, it would have been easy for the British establishment to look to re-ingratiate itself with the US and send troops to Vietnam, as indeed it had done a decade earlier in Korea. While Wilson didn't actively oppose the Vietnam War when he became Prime Minister in the way he had beforehand, he ensured that British forces were never sent to Vietnam and the UK was not bogged down in the horrific quagmire in South East Asia.

What the Chilcot report offers is the chance to assess where some of the responsibility lies for the decision to invade Iraq – a decision even more disastrous than Suez – and why Blair didn't steer the more sensible course of his predecessor Wilson. We already know why, fundamentally. As my colleague James Macintyre writes, "9/11 is the crucial context to the Iraq war because it is believed that Blair decided on that day to commit to doing whatever Washington wanted, effectively handing British foreign policy to the US. Initially the British prime minister sought a second UN resolution and pushed for Europe to come on board but when it became clear this was not possible he pressed ahead anyway."

There are two related but distinct ways in which the 2003 Iraq war was an unmitigated disaster. The first pertains to the victims – the Iraqi people and coalition troops – the second to the impact on British politics since then.

Having been under the iron fist of Saddam Hussein since the 1960s, the Iraqi people had suffered a great deal. A cruel dictator who infamously slaughtered his own people, Saddam's war against Iran in the 1980s saw him cast as a friend of the West. This changed when he threatened Western oil supplies by attacking Kuwait in 1990. The First Gulf War swiftly pinned Saddam's forces back and left Iraq to fester under sanctions for the next 12 years.

Though they had already experienced hard times, the invasion saw internecine conflict claim life and limb on an unprecedented scale. There is no way of knowing how many people died because of the war. There are, however, various estimates. A report in 2013 suggested over half a million Iraqis had perished because of the US/UK incursion and occupation. An earlier report published in The Lancet suggested 655,000 civilians had died in the war and its aftermath. Another report offered a lower figure of 116,000 civilian deaths.

What is clear is that immense suffering has been endured in the aftermath. The failure to secure the country after the invasion and the calamitous decisions to disband the Iraqi Army and dismiss Ba'ath party members from public roles were among the contributory factors to the chaos.

What the figures above don't take into account is the impact of the bloody civil war which has been intensified in the past few years by ISIS. Just this week over 200 people were killed in a major bomb attack in Baghdad – the gaping chasm of leadership and security left by the removal of Saddam led directly to the civil war which has raged ever since.

Make no mistake, Saddam was a brutal dictator – but without his removal in 2003, it is extremely unlikely ISIS would have developed as rapidly as it has. Before 2003, Al-Quaeda had little presence in Iraq. After a war that was supposed to make us safer, Al-Qaeda was rampant there. Before 2003 Sunni-Shia conflicts were acrimonious and sometimes deadly, afterwards, it was all out civil war between them.

Along with the war in neighbouring Syria, the conflict in Iraq has not only cost countless Iraqi lives, it has contributed to widespread instability in the region which has created the refugee crisis and emboldened terrorist attackers in Western cities.

The 2003 invasion was also the start of an almost unprecedented exodus of Christians from Iraq. In 2003 there were 1.5 million Christians there. Now there are fewer than half a million. That this tragic situation was precipitated by two Christian leaders (Blair and Bush) is a grim irony. Read the stories of some of those forced to flee with family members dead and maimed. Weep that this war turned into a war on Christians.

The impact on the Iraqi people has been vast, but the impact on British and American service personnel has also been profound. 179 British troops were killed. Help for Heroes suggests there are 75,000 personnel recovering from injuries sustained in Iraq as well as Afghanistan. The US bore the brunt of the deaths and injuries, with over 4,400 killed and over 30,000 injured. For every one of these statistics, there is a person dead or injured, a family bereaved or altered significantly.

If the first colossal impact of the war was on the Iraqi people (and coalition troops), the second was on the British political scene. This impact too, was profound. While Labour won the first General Election after the invasion, in 2005, it has not done so since. The Party, torn apart by many differences, has fixated on Iraq and its legacy. While some still believe invading Iraq was the right thing to do, others abandoned the party almost immediately. The various tribes which make up Labour are more divided than ever as the recent attempt to remove anti-Iraq War leader Jeremy Corbyn has shown. There's a chance he wouldn't even have been in control of the party, were it not for festering sentiment over the war, which powered the movement which swept him to office.

British involvement elsewhere in the world has been seriously affected by the disastrous impact of Tony Blair's decision to invade Iraq. David Cameron's 'hit and run' approach to Libya in 2011 showed that he'd learned the lesson that British troops on the ground would be very unpopular. So Colonel Gaddafi was removed simply by air power. There was another lesson of Iraq that hadn't been learned, though – a lack of planning for the aftermath means Libya remains in a state of chaos – a breeding ground for ISIS and a starting point for many dangerous refugee journeys.

Reluctance to see British troops involved in Syria can be directly traced to the unpopularity of the Iraq War. Humanitarian intervention of the type championed by Blair in Sierra Leone would be incredibly hard in the post Iraq climate. In short – Iraq has poisoned the national debate over whether it's ever right to commit our troops into harm's way, because of the cavalier way they were deployed in Iraq without a plausible exit strategy and without a proper mandate. Chilcot said today that war was "not a last resort". The UK decided to invade before the peaceful options had been exhausted.

Hundreds of thousands of people are dead. The Middle East is in flames and there seems little prospect of Iraq or Syria becoming calm any time soon. The concept of humanitarian intervention has been dealt a possibly fatal blow. Pre-war claims have been exposed as foolish. Post-war fragments of a country are shattering into smaller, even more brittle pieces. Blair and Bush let slip the dogs of war. What followed was sheer havoc.