If Jonathan Powell was half as straight-talking inside the Blair tent as he was in his Guardian interview last week, it makes the central mystery of the Iraq war even odder. Here is someone who advocates speaking to terrorists, understands how poor intelligence can be, and admits to errors, who was at Tony Blair's shoulder for 13 years. He comes across as level-headed and dryly humorous.

In saying that Blair could be a "bit of a flippertygibbet" and was neither bold nor radical enough in office - because he feared losing it - Powell also seems to be a man prepared to speak uncomfortable truths. Asking around, the general view is that he did behave just as forthrightly when it was the inner circle. He admires Blair, but he was no passive courtier.

Five years on from the opening salvoes of the Iraq war, and 10 years on from the Good Friday agreement, it's a perfect time to revisit the strange symmetry between brave, dogged peace-making in Northern Ireland and catastrophic war-making in the Middle East, which will always define the Blair government. Powell's testament is only part of the story. We haven't had a frank reassessment from Blair himself. Perhaps we never will. The layers of self-justification, like scar tissue, are already too thick. Maybe the truths are just too painful to look at clearly.

Nor have we had frank, no-holds-barred retrospectives from the rest of Blair's inner circle. Powell was clearly vital to the peace making part of the story. It became personal for him, and he battered away at it. Yet what also emerges from his account is that Blair's self-belief and brass-necked optimism was the defining factor. The deal was not always inevitable. In a way, if Blair had been better educated about Northern Ireland, better educated about history, he might not have been as bold. The bloody history and entrenched attitudes had scared many wiser men away. But with his grin and faith he kept going.

Put it like that, and the Northern Ireland/Iraq paradox starts to disappear. Boldness? Not knowing too much history, and being ready to ignore cautious advice from experts? Charging ahead, armed with soundbites and timetables?

The more one hears about what was really going on inside the Blair camp, the more one realises that none of the aides or friends did more than briefly deflect Blair's own instincts - not Powell, not Alastair Campbell, not Peter Mandelson, not Derry Irvine. "Sofa government", was less about handing cabinet power to a small, unelected clique, as it seemed at the time: it was a cover for the Blair presidency, something worryingly close to one-man government. Only Gordon Brown, with his rival powerbase at the Treasury, could deflect Blair, and then only because of regular stand-up rows and at great mutual cost.

Back to the Iraq war decision. Britain had in the Foreign Office a priceless source of information and advice about the region. In Robin Cook there had been a foreign secretary who knew his falafels from his onions, and understood the perils - enough so for him to resign from his next cabinet job. Jack Straw, foreign secretary at the time, admitted later to grave worries. Blair was getting, at the very least, mixed messages about his legal position. His diplomats were telling him the truth about international support. Among Labour dissenters, Liberal Democrats and some Tories, there was an eloquent, detailed alternative view being expressed in the Commons, never mind the great demonstrations outside.

So why did he not pause? Why did he not properly think about what might happen in Iraq after the invasion? Why did he not have the conversations with George Bush that so many in London and Washington expected him to have? Powell's testament, as a pro-war adviser, suggests that it was not, really, about a lack of advice or simple ignorance. No, it was about personality and temperament - Blair's. The very things that brought him success in Northern Ireland, brought him disaster in Iraq.

I think the two anniversaries are even more closely connected than that. It was because of the Good Friday agreement that Blair's self-belief and impatience with facts about history and the dangers of meddling were super-charged. War-making and peacemaking are perhaps not so different, from the perspective of a prime minister. Both require decisive, bold actions. Both mean shutting your ears to the voices of caution.

This hubris meant the wrong lessons were learned from the Balkans. The former Yugoslavia and Iraq were patchworks of peoples, held together for a while by tyrannical central power, when it was convenient for the world order. Pull away the centre, and you unleash civil war and ethnic cleansing. Far from Kosovo emboldening Blair, it should have terrified him.

In this tale of hubris and nemesis, now emerging in full technicolour through the memoirs of the participants, is there anything we can learn for the future? That it is nobler to make peace than war, certainly. That it is crazy not to listen to all the storehouse of advice and wisdom a government can get its hands on. But most of all, that "never" never means never.

Powell's most provocative argument is that today Britain should be keeping lines open to Hamas and even al-Qaida. It was rubbished by No 10; but I wonder if that wasn't more because of irritation about the messenger. For he is right. There is no moral distinction. Britain was quietly talking to the IRA when it was determined to have a united Ireland, by killing as many people as it took. Al-Qaida's united caliphate is a bigger project, and a deeply illiberal one, but it is not logically so different. The Provos had plenty of silent supporters in Britain, just like al-Qaida. Then too, for a long time, there seemed nothing to talk about.

Until one day, suddenly, there was. You "never give in to terrorists" but you can never finally defeat them either. The reason the "war on terror" has been such a dangerous phrase is that you can never win a war on an abstraction - "terror" is always out there, and inside us. So the war can never end. In fact, the war is with networks of human beings whose views are radical and dangerous but who, one day, will have to be talked to and offered compromises and a way, as Powell put it, to get "off the hook".

We may be years or decades away from that. When it comes, it will be a difficult, tiring, seemingly impossible job, but those who do it will win their place in history. War-making is a much lesser thing. Powell clearly understands that. If he had not, then as the man in the shadows, he'd be publicising a book on "Me and the Iraq war", not "Me and the Irish".

jackie.ashley@theguardian.com