The Iraq discussion was for many years the dullest in British politics – dull because the question “Why did we invade Iraq in 2003?” has such a straightforward, decisive and (to be frank) obvious answer but so few folk want to accept it.

We invaded Iraq in 2003 because we believed Saddam Hussein had spent the previous decade defying us – attempting to build nuclear and chemical weapons plus a super-gun to fire them – and, in the post-9/11 world, we could not allow him to carry on defying us.

Many readers may start going on about dodgy dossiers or 45 minutes or whether WMD were found. That’s all irrelevant. We didn’t invade Iraq because we believed he actually had WMD at the time (though we may have believed that). It was common to speculate in 2002 that any sane leader would have moved any WMD out of Iraq after 9/11. His having them or not having them then was of only marginal interest.

Others say there was no link between the invasion of Iraq and 9/11. Depending upon what is meant, that is either irrelevant or absurd. It is irrelevant in that Saddam’s defiance in the 1990s was long before 9/11, so nothing per se to do with it. But it is absurd to claim the Iraq invasion did not follow from 9/11. Everyone understood at the time that it did. After 9/11 the only real debate was whether we would invade Afghanistan and then Iraq, or Iraq first and then Afghanistan.

During the 1990s, believing Iraq to be in violation of the requirements imposed upon it that came in lieu of toppling Saddam in 1991, there were extensive sanctions imposed upon Iraq, a no-fly zone and repeated aerial bombing campaigns. We did not invade only because of American reluctance to commit ground troops (a big issue in the former Yugoslav Republic conflicts of that era). The British were not reluctant participants in the military conflicts of that era – dragged along to participate in “American wars”. We were the chief belligerents, eager to drive the US ever on and stiffen its sinews.

After 9/11, the US learned that isolationism and a reluctance to get too involved had not yielded the “peace dividend” it had hoped, but, instead, the deaths of thousands of American civilians on US soil. The bar on ground troops was removed and top of the list, once Afghanistan was done, was the chief defier of the international community: Saddam Hussein. That is the 9/11 link.

Some claim the fact no WMD were found in Iraq vindicated the French-German-Russian position. But it did not. Their position was that Saddam could be contained, not that he had not been defying us. It was not a serious position.

As for the British anti-war marchers, they reflected the very worst of British political instincts. They were selfish – preferring thousands of Iraqis to die from sanctions or from Saddam’s oppression to our becoming involved in a land far away of which they knew nothing. They were isolationist – preferring evil to flourish in the world to the Good Guys acting. They were relativist – asking “Why should Iraq not have nuclear weapons anyway?” They were cynical – claiming there was some conspiracy to take Iraqi oil. And they were wrong – like all the rest of us, they too believed Saddam had been defying us. The difference was they thought he should be allowed to get away with it.

We can ask in hindsight (which is oft a poor-because-infallible guide) whether it has not turned out that Iraq was a mistake, because it turns out that in fact the one point upon which all were agreed – namely, that Saddam had spent a decade defying us – was wrong. In truth, it appears now he had only been pretending to defy us and somehow we and our intelligence services were all taken in from 1993-2002. That is a scandal.

But it makes no difference to why we went to war given what we all thought we knew at the time. There is no mystery about that at all.