IT WAS only a phrase, a snippet from a long speech. Yet for some reason Tony Blair's words have nibbled away at me for a week. What Mr Blair said was: "...let there be no moral ambiguity about this..."

The Prime Minister was speaking at the Labour conference in an address much praised at the time. He was statesmanlike, which is good - until you remember that it is usually statesmen who get us into wars. But we'll let that pass.

The selected phrase preceded these words: "Nothing could ever justify the events of September 11, and it is to turn justice on its head to pretend it could." That much is fair enough as no creed or cause could excuse such an unspeakable act. The terrorist atrocities in New York and Washington marked one of the bleakest days in modern history and the ensuing crisis is the most alarming many of us have lived through.

These are the gravest days since the Cuban Missile Crisis of 1962, or so I keep reading. That crisis went over my head at the time but I was only six.

For a few weeks in October - what have us Librans done to deserve such a war-like month? - the world wobbled when the USA discovered Soviet nuclear missile sites in Cuba. After a stand-off between Kennedy and Khrushchev, and a naval blockade of the Caribbean island, the Soviets backed down.

It was easier to spot the teams in those days. The Yanks versus the Ruskies. There was no confusion. In the present crisis it should be simple too: it's us versus them; right against wrong; good versus evil; West against East. Or something like that.

This is why Mr Blair's words worry me. I don't like to be told that there is no "moral ambiguity" about the war on terrorism and blasting Afghanistan to bits - however appalling the provocation. What's alarming is this: ambiguity, moral or otherwise, is where the room to move and think lies. Without this leeway, we only have what, to put a backspin on Mr Blair's words, you might term "immoral certainty".

All these handy labels can mislead, especially "war on terrorism". For a morally unambiguous campaign, there is an alarming vagueness about this phrase. Are we talking about all terrorists or just those in our sights now? And can we be certain that raining missiles down on Afghanistan will root out Osama bin Laden and his Taliban hosts? Might we not be playing bin Laden's perverse game, turning him into a Muslim folk hero and rushing into an unstoppable war of West against East - just what bin Laden wants?

Of course, there isn't a sacred shred of moral ambiguity in dropping bombs and food parcels into Afghanistan. I hope the airborne sustenance came with a covering note: "Dear Inhabitant of Afghanistan... if you have not been blown up, please accept this meal on behalf of the United States." Who says the Americans don't understand irony?

In preaching against moral ambiguity, Tony Blair in effect stifles disagreement, making open debate seem treacherous. But at least he has the fulsome backing of this country's most highly-paid columnist.

In The Sun, Littlejohn - these days, he's too lofty for a first name - lays into the "anti-American fascist Left", an abusive catch-all covering anyone who doesn't think that Bush and Blair are not absolutely right in everything they do. So he'd probably hate a worrying, liberal doubter like me.

Oh, it must be a wheeze being Littlejohn. All that money and a simple, black-and-white, unambiguous world right outside your window.

Nice work if you can hack it.