Once again, the armchair generals of the Blair Memorial Battalion stand, or rather sit, ready to serve.

You have to grant them a few points for tenacity. When you seek to make amends for past catastrophes simply by re-running the entire awful movie, you mark yourself as thrawn. Or perhaps just very stupid.

The renewed chorus of those demanding "intervention" in Iraq has been led, of course, by Tony Blair himself. A lot has been said, most of it derisive, about his remarks on the Islamic State of Iraq and the Levant (Isis) insurgency and the west's responsibilities, but a couple of aspects are worth a little study.

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First and obviously, Mr Blair attaches no culpability to himself. In his world, nothing happening in Iraq in 2014 has much to do with the decisions he and George W Bush took in 2002-2003. In the latest version of the former prime minister's never-ending war story, a lack of "action" against Assad's Syria - where Isis was ranged against the regime - somehow explains the fragmentation of Iraq. Let's call that disingenuous.

It has seemed clear enough, equally, that Mr Blair operates from an assumption. Few properly remember what went on ten, 11 or a dozen years ago, he thinks, and fewer care. His own assiduous attempts to cloud every memory have been dedicated to that outcome. So today his diminished band of followers, with reputations of their own to retrieve, will tell you we "had to topple Saddam". Mr Blair told the Commons more than once Britain had no interest in regime change. Who remembers?

Barack Obama remembers. A large part of his first presidential campaign was based, after all, on a promise to clear up the hideous mess in Iraq created by Mr Bush, Mr Blair, and their supremely arrogant, blithely inept invasion. The aftermath of that turned out to be far bloodier and more costly than the war proper. But in the fantastic circular logic of Mr Obama's neo-conservative American critics, and of their British camp followers, this allows only one self-serving conclusion: "we" should have stayed in occupation for longer. How long? They'll get back to you.

Revealingly, these characters, these champions of democracy, give not a damn about public opinion, here or in the US. They gave no sign of caring for what their people thought in 2003, either, a fact that might explain their addiction to deceit, then and now. So the people, certain to answer the cry "Back to Iraq!" with an expletive or two, are omitted from the grand arguments. Just like last time.

The attempt to shift the onus of responsibility on to Mr Obama (and to a lesser extent David Cameron) amounts to a spectacular sleight of hand. That the US President likes to do his killing with anonymous and cost-free (for him) drones is well-known. That he does not often act quickly or decisively has been a mark of his presidency. In neo-con language, these traits signify "weakness" - as though strength got us anywhere - or even cowardice. So a president's desire to prevent his country falling headlong into the same old traps is used against him by those who fashioned the traps.

If your memory or your reading stretches back to the beginnings of the Vietnam debacle, you will probably shift uneasily at the news of 300 US special forces "advisers" being sent into Iraq. An aircraft carrier - with a touch of black comedy, the USS George HW Bush - with its four squadrons of F/A-18s could count as another sign that "mission creep", the steady expansion of military involvement, cannot be far behind. But Mr Obama seems, by his standards, resolute: US combat troops will not return to Iraq on his watch.

The government of Nouri al Maliki, which has lost the confidence of its western patrons, will not be encouraged by such declarations. It is already aggrieved its dial-an-airstrike privileges have been suspended, at least until such time as Mr Obama sees no alternative and can identify targets which do not turn out to be the usual groups of helpless civilians. The White House understands the problem even if Iraq's desperate and beleaguered government does not: how would the Sunni people of the country respond to American intervention?

Some 4500 US personnel died in the nine years following the glorious Bush-Blair liberation of Iraq and the dismantling of the Ba'athist state. They died fighting the people upon whom they were, according to the politicians of the west, bestowing democracy. Even the great majority who did not take up arms against the occupiers seemed to have no taste for occupation and the insurgency it provoked. At the start, the west blamed Saddam "hold-outs"; these days we have vicious Isis jihadis. The simple truth remains: for every intervention, a reaction.

Estimates of the fighting strength of Isis vary. One puts its number at little more than 10,000. If nothing else, that's a measure of how weak the Maliki government was before this crisis, and of how easy it would be, on paper, for the US to inflict its military strength. On the battlefield, 2003's "Operation Iraqi Freedom" - yes, truly - was also a bit of a walkover. Saddam's vaunted armies were beaten in a week and finished in three. But then what happened?

Should one American bomb fall, Isis will take strength. Among Sunnis in a country collapsing into its old, pre-British contours, western intervention will only draw recruits to those ruthless fundamentalists. Mr Blair's insistence we must fight such people in Iraq (or anywhere else) to keep them off our own streets is another of his perennial excuses for perpetual war. It does not solve the fundamental problem for the west and Mr Obama: intervene and resistance follows. The flag of resistance matters less than the fact.

The US President would like to see a new government in Baghdad. That's understandable. Mr Obama would like the Iraqis to find an adept democrat who could resist the temptation to sectarianism, bind Shia, Sunni and Kurd together, and remember his obligations to his patrons. Once again, the echoes of Vietnam, when the US was never done trying to find the ideal leader who wasn't too corrupt yet capable of keeping the show on the road, are loud. In Iraq, as in Vietnam or Afghanistan, it doesn't work.

For Mr Obama, the test now is to stick with the strategy he has chosen. If he is swayed by those shameless enough still to be peddling arguments long since refuted amid blood and slaughter, he will make a mockery of what remains of his reputation. Intervention, "liberal" or otherwise, is neither a moral right nor an obligation. Its claims must be verified.

In the case of Iraq, those claims have to be tested, surely, against experience first and foremost. They also have to be measured against the certainty that people don't care for being bombed at the behest of chickenhawks far away who never manage to put themselves in harm's way. That they also ignore the democratic principles they claim to defend is a fact of which Mr Obama seems, mercifully, well aware.