It seems such a tiny, insignificant thing. Why worry about planting a little propaganda and bribing a few journalists when your men in the field are dying day after day? "This is war," says the Pentagon. Yes indeed, adds the sonorous senator who chairs the armed services committee, "this is war". And in war, of course, anything goes (even including bombing al-Jazeera) because ... well, it's war, isn't it?

So two linked stories rise, then fade away. Maybe there was outrage last week when the LA Times reported that the department of defence had hired a Washington company called Lincoln Green to harvest phoney tales of triumph written by US army personnel in Iraq - then, duly translated, feed them back to the "free" Baghdad press Donald Rumsfeld loves so much. Maybe eyebrows flickered when it emerged that some Iraqi reporters and editors are on America's payroll. Maybe the Arab world was still fuming over alleged threats to its favourite 24-hour news channel.

But, war changes - and excuses - everything. Events move on. We slide swiftly back to a status quo where politicians seek to make the media the villains of every piece. Re-enter Alastair Campbell, singing his greatest hits (against the BBC). The director of al-Jazeera arrives in Whitehall demanding to see Tony Blair, who turns out to be somewhere else. Dirty, dud yarns don't seem to matter a jot.

Keep the outrage pot bubbling a moment longer, though. Do you remember Armstrong Williams, TV frontman and syndicated columnist? The Bush administration handed him $240,000 under cover of darkness to plug its education reforms. Do you remember how the American government thereupon funded a string of superficially independent news "reports" on its education and energy reforms in video packages that small stations just plonked on air as all their own work? What about the latest public broadcasting ruckus, with Karl Rove, Bush's fixer, discovered chatting secretly to PBS chiefs about starting a neo-con talk show and getting a few more Republicans on studio duty?

None of this has anything to do with "war" (unless it be some undeclared war on truth). But it is all part of an inescapable pattern, one so serious it can't be allowed to fade away.

Journalists protest too much about spin. There's no great British purity to a system that wallows weekly in tales about who briefed whom on pension reform and the like - one that makes Conrad Black a peer of the realm for services to Mrs T. There's scant purity, either, in an American way that brings packs of favoured journalists into the inner sanctums of the White House with every change of administration and allows the presidential elections to be fought against a background of promised liberalisations of media ownership worth billions to big guys in the back offices.

Nevertheless: some lines can't be crossed, some boundaries of conduct have to be defended. And pushing bundles of dollars under the counter towards journalists of any persuasion in order to buy complaisant coverage direct is one stark frontier too far.

Didn't the Bush team see that when the Armstrong Williams scam was exposed? Didn't it realise that grime sticks to the giver as well as the receiver? Apparently not. Apparently, its moral compass has gone on permanent blink.

Voters, in Iraq as well as Iowa, are grown-up, sentient human beings. You can tell them the truth - that democracy is a rough-and-tumble way of governance - but you cannot foist lies on them in the name of freedom.

Suppose, for the sake of hope and argument, that one fine day the insurgency does begin to fade, that Iraq's economy revives and the first fruits of transition begin to appear. Who'll bring that news and that understanding to the people but the journalists of Iraq, professionals enduring constant peril day by day? But no one, surely, will pay them heed when they begin to bring good news. They will be dismissed, with a shrug, as paid lackeys from an occupying power, hacks who would say that, wouldn't they?

And exactly the same problem infects the al-Jazeera debacle, this time for Blair as well as Bush. The BBC World Service is closing down in much of eastern Europe in order to spend £19m of Foreign Office cash a year on an Arab TV news channel to rival al-Jazeera and bring the western message to the streets of the Middle East. But what are those streets now asked to think? If you can't beat them, bomb them? If you can't woo them, sue them? This isn't just wrong, whether war or not. It is imbecility.

p.preston@guardian.co.uk