This basic question - of when war is justified - has puzzled philosophers for centuries. It came into sharp focus this week, with the apparent leaks from within the intelligence community, that Tony Blair's PR advisers had manipulated the truth and the media in a bid to boost public support for a war that should never have been.
It was given added impetus by an outspoken attack from former cabinet minister Clare Short, who claimed Blair had duped the public into supporting a policy he had determined in secret with George W Bush up to a year ago.
Given his reluctance to spell out in detail the case against Saddam Hussein before sending troops off to battle two months ago, Blair's response - that those making serious allegations 'should back them with evidence' - prompted more than one headline writer to quip: 'Precisely, Prime Minister'.
And yet, notwithstanding the obvious point that a lot of the security services' work should remain confidential, there is a legitimate question to be asked about just how the notion that Iraq's dictator was 45 minutes from launching WMD came into being.
Tory requests, therefore, that this whole saga should be the subject of a public enquiry - even if they are motivated by something other than a selfless quest for truth - should be granted. In the context of unminuted meetings, unattributable briefings and conflicting claims, it may never be clear who is on the side of the angels in this latest PR debacle, but the effort is surely worthwhile.