Fred Astaire said it best: "How can you believe me when I tell you that I love you when you know I've been a liar all my life?"
Or, what possible reason is there for believing that the Syrian regime has used chemical weapons against its own people when we know we were lied to about weapons of mass destruction in Iraq?
It is now beyond argument that the flat-out assurances from Bush and Blair that Saddam was sitting on an ominous arsenal of WMD were deliberate lies designed to lure us into supporting the invasion and occupation of Iraq.
And we know now, too, that the confident predictions from the same sources that the people of Iraq would hang out the flags for the invaders turned out to be the opposite of the truth. The same people of Iraq – or, at least, those who have survived – are still paying the price.
Very few of the commentators – if any at all – who rubbished the warnings of anti-war campaigners that things would turn out more or less exactly as, in the event, they have turned out seem to feel any need to apologise, or admit they were wrong.
This must be of great comfort to Obama, Cameron and Hollande as they ponder escalation of their armed interventions in Syria.
Here they go again, insisting in tones of dogmatic certainty that the Assad regime has used chemical weapons against civilians.
"Crossed a red line," says Barack Obama. "Leaves the world in no doubt [about the need to oust Assad]," chips in David Cameron. "There is no doubt that it's the regime and its accomplices [who have used chemical weapons]," offers French foreign minister Laurent Fabius.
Carla Del Ponte disagrees. She's the head of the UN commission investigating the use of chemical weapons in Syria.
She said last month: "Our investigators have been in neighbouring countries interviewing victims, doctors and field hospitals ... There are strong concrete suspicions of the use of sarin gas ... this was used on the part of the opposition, the rebels, not by government authorities."
Del Ponte didn't assert as fact that the rebels were using chemical weapons, or say for certain that the regime was not. What she said was that the evidence which the UN had been able to gather cast greater suspicion on the rebels than on the regime.
She had declared in February that both sides in the Syrian conflict had been guilty of war crimes and should be brought to justice. She should know a bit about war crimes and bringing perpetrators to justice.
Del Ponte is a former Swiss attorney general. She was chief prosecutor at the International Criminal Tribunal for the former Yugoslavia (ICTY).
She headed the team which assembled the evidence against former Serbian leader Slobodan Milosevic and then led for the prosecution when the case came before the ICTY at The Hague.
Not everyone was pleased with her handling of the Milosevic case. Civil liberties groups and commentators from both left and right condemned the trial as "victors' justice" and argued that the prosecution evidence was contradictory and unconvincing.
Among Del Ponte's sharpest critics were hard-right US politicians, including the neo-con evangelist Pat Buchanan and Ron Paul, the man who made Mitt Romney look like a liberal, when he campaigned for the Republican presidential nomination last year. But Del Ponte, unimpressed, pressed on. No soft touch, she.
None of this has prevented the US, Britain and France from continuing to declare as flat fact that the Assad regime has used chemical weapons, while denying, or simply ignoring, the evidence of the rebels' use of the same deadly devices.
Here we go again. Next stop Iran. (Is it worth pointing out that there is no evidence whatsoever that Iran has a nuclear weapons programme? I suppose not.)
The common characteristic of the dictators that Obama et al are willing to use force to overthrow is that they won't be told what to do by Obama et al.
But feudal regimes, like Saudi Arabia's, where there is no freedom of religion, women are sorely oppressed, homosexuality is outlawed and it is a criminal offence to try to organise a political party, are well onside and perfectly safe.
Alan Jay Lerner, who wrote the lyrics of the Fred Astaire classic quoted above, also wrote Gaston With Flowers, from Gigi:
Through the sleet and drizzle/ You can hear the sounds of soldiers.
The Kalashnikov and splutter/ On a sunny day.
The sentiment was audible in Enniskillen on Monday. But only if you were listening.