Why did Tony Blair go to war in Iraq? There are two conflicting answers to the question that are widely accepted, one outlined with passion by Blair’s critics and the other made with equal intensity by some of his supporters. Both will be aired relentlessly now that John Chilcot is finally publishing his report. Yet neither makes any sense.

Blair’s critics argue that he lied persistently and in doing so became a war criminal. This answer has fuelled mistrust in politics to stratospheric levels. If Blair lied to send soldiers to their futile deaths it is hardly surprising that in recent months voters did not believe a prime minister warning that the consequences of Brexit would be dire.

But step back and think about this answer for a moment. Blair came to power in 1997 obsessed with securing the voters’ trust. At one point he suggested that even the perception of mistrust would merit the resignation of a minister, an absurd proposition that if enacted would have triggered his own departure by the autumn of 1997.

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Nonetheless Blair knew Labour had not been trusted with power for 18 years and he was going to do all he could to build an unyielding bond between his government and the electorate. He published annual reports on what his government had promised and achieved, apologised for mistakes, gave press conferences that lasted forever so any tough question could be asked, and, in the early days, made only incremental promises so there could be no accusations of betrayal. Leaders change in style and outlook, but they do not change from being gripped by the necessity for trustworthiness to being manically indifferent to it.

There is also the wider issue of character. Blair pursued the Northern Ireland peace process around the clock. Why would someone allegedly indifferent to the bloody consequences of war work sleeplessly to secure peace? The theory does not add up.

But nor does the one espoused by Blair’s more devoted followers and by Blair himself. According to this account he ceased to be bothered by popularity and believed only in “the right thing to do”. Millions may have marched against the war but he was an evangelical crusader. Popularity and his previous expedient searches for a third way had to be nobly jeopardised and replaced by the application of unyielding conviction.

Once more, this answer asks us to accept that a leader can change utterly, a metamorphosis that psychiatrists would struggle to explain. Here was a pragmatic leader who had always navigated a third way around challenges, who had built up a broad base of support and consistently reminded his party that nothing could be done without winning elections. Was he willing to blow everything on the basis of a suddenly acquired passion for Iraq and the Middle East? This also makes no sense.

The answer to the mystery is rooted in a political context, one ignored by previous investigations and one that will almost certainly be underplayed by Chilcot. The first part of the answer is to ask the right question. It is not the one that misleadingly opens this column. Blair never had to answer the question: should the UK invade Iraq? He had to answer a different one: should I support President Bush who has decided he wants to remove Saddam Hussein?

Facebook Twitter Pinterest Tony Blair and George W Bush: ‘Blair told visitors to No 10 that one of his objectives was to prove that a Labour prime minister could work with a Republican president of the US.’ Photograph: Stephen Jaffe/AFP/Getty Images

Given Blair’s political past and character there was always only going to be one answer to that question. Blair had been brought up politically in the 1980s when Labour lost elections partly because it was seen as “soft” on defence and anti-US. When Blair came to power in 1997 his words outside No 10 were as much about a rejection of his party’s 1980s past as they were about the future. “We were elected as New Labour. We will govern as New Labour,” he declared revealingly and defensively.

US presidents did not approve of 1980s Labour. New Labour would be close to US presidents. At the start of his second term in 2001, before the attacks on September 11, Blair told visitors to No 10 that one of his second-term objectives was to prove that a Labour prime minister could work with a Republican president of the US. Brought up on defeat he was neurotically worried that the Conservatives were forming close ties with Republicans in Washington. In his conviction that New Labour must be different he moved towards his doom.

But he did so in a way that was wholly in keeping with his political character and was not some insane leap to a new personality. He sought a third way. He persuaded Bush to go to the UN, knowing that if the invasion was backed by UN resolutions he would keep a big tent of support in place in the UK and constrain the wilder elements of the Bush administration.

In persuading Bush to go to the UN he pledged UK support for military action. The third way UN route meant that Blair had no choice but to frame the argument for the invasion solely in terms of Saddam’s WMD and UN resolutions. For once, his third way was not an escape but a trap. When he became unpopular he also had no choice but to make a virtue of his shrinking big tent by becoming a leader who “does the right thing” irrespective of popularity.

In his conviction that New Labour must be different he moved towards his doom

There is no point guessing whether or not Blair believed the hopelessly speculative intelligence or not. He had to believe it. He was putting a case in a desperate attempt to persuade parliament, the electorate and the media. Of course he genuinely saw a case for war. No human being could do what he did without believing there was a case. But in different political circumstances Blair could have applied his forensic intelligence to recognising the case for not going to war.

Blair’s course, like David Cameron’s over the calamitous referendum on the EU, raises issues about prime ministerial depth and experience but not about integrity. Blair became prime minister with no experience of government and without having to explore policies beyond their electoral implications. His main guide was his party’s vote-losing past and that was nowhere near enough.

Cameron, also brought up on his party losing elections and no ministerial experience, was in some respects a similar leader to Blair. Both were well intentioned but when faced with momentous challenges were out of their depth in a way that will have bleak implications for the UK and the rest of the world for a long time to come.