If allied planes were mistakenly responsible for yesterday's far deadlier attack on a convoy containing refugees in southern Kosovo, the lesson will be even more bitter.
In this war, as in every war, propaganda is a vital weapon. It is vital for the Western democracies, where strategies ultimately depend on public opinion - but it is also vital for Belgrade. By fast public relations footwork, Nato has neutralised the train disaster. If confirmed, the slaughter of refugees - 64 ethnic Albanians, according to the Serbs - may be far harder to cope with.
Convince the public the cause is just, liken the enemy to Hitler, sanitise every nastiness perpetrated by your side, and claim you are firmly on the way to victory - these are the rules of the game. Both sides have been faithfully playing them. Until the train on the bridge at Leskovac.
The affair could have been a PR disaster, proof of how the allies in their frustration at their failure to land a knockout blow to the Milosevic military machine were sinking to terror bombing of helpless civilians. Prevarication, or pretending the train was a legitimate military target, would have made matters even worse.
Instead, we witnessed a rare and deliberate show of honesty. It was an "unfortunate accident... we are all very sorry for it", Nato's Supreme Commander, General Wesley Clark, said as the press was shown a full cockpit video of the incident. The pilot, it transpired, had fired not one bomb, but two; the second was fired after he knew he had hit a train. Quite why that second attack happened is unclear. But we basically know what went on, and few more questions need to be asked. If it was allied aircraft that slaughtered the refugee near Djakovica, we must brace for a far more harrowing mea culpa from Nato - and, conceivably, a sea change in public attitudes to the air war.
Until now, Belgrade's forays into the propaganda war have been more comic than threatening: "88 Nato men dead and 32 planes lost", ran a front- page headline last week in the Greek daily Athinaiki, gleefully reported by the official Yugoslav news agency, Tanjug. News indeed to warm Serb hearts, but somewhat at variance with the one verified loss of an F-117.
Indeed, two mysteries of this strangest of wars are the tiny scale of Nato losses (one aircraft and, as far as can be ascertained, not a single life - astonishing given the advance publicity about Serb air defence capabilities) and the small civilian death toll in Serbia caused by the bombing - until yesterday, at least, under 100. Had there been more, we would certainly have known.
Then there have been the tales put out by the Serbian Information Ministry of "columns" of deserters from the Nato force stationed in Yugoslavia, donning civilian clothes and streaming into Greece and Bulgaria, and of conscience-stricken Norwegian pilots who refused to take part in the air attack, and returned home to the cheers of their countrymen. For those who dare criticise, they must ponder the fate of Slavko Curuvija, the independent newspaper owner shot dead outside his Belgrade apartment on Sunday.
Nato, of course, propagates its own brand of wishful thinking. Early on, we were told of Kosovo Albanian leaders who had been murdered. It transpired they had not and Nato, admittedly, retracted the claim. On Sunday, its spokesmen were claiming to detect fissures in the Yugoslav army's high command over President Milosevic's policy in Kosovo. The next day, despite every sign that the war is solidifying support for Milosevic, the Secretary of State for Defence, George Robertson, was virtually predicting a coup.
But these are small errors, set against the giant shadow cast over everything the allies say by the miscalculation with which the war started. A few days, they said, just a handful of cruise missiles, before the dictator came to his senses.
"We always knew this would be a long haul," Nato now insists. If so, then everything it implied, if not said aloud, before 24 March was nonsense. But did not the First World War begin with the blithe conviction that the boys would be home for Christmas?
So far, Western public opinion doubts about the air war strategy have been submerged by the flood of horror stories of pillage, rape and murder committed by the Serbs. But these stories, too, could in turn be submerged by the horror of what apparently happened near Djakovica.
For the Serbs, the carnage was a propaganda coup: a "crime against humanity" according to the Foreign Ministry in Belgrade - exactly the language used by Nato leaders to describe Mr Milosevic's tactics.
The attack strikes at the heart of the weakness of Nato's presentation of the war - not lies over daily events, but the fog of confusion and contradictions surrounding the war's ends. Suddenly Kosovo is acquiring the reek of Vietnam, and of the Pentagon's line then - which summed up the futility of its strategy - that to be saved, villages had first to be destroyed.
"We will win the war," sayBlair, Clinton et al, day after successive day of bombing targets that never seem to be destroyed the first time around. Ah, but the bombing is working, they insist, only to order another 300 aircraft to finish the job . Defying, if not the truth, at least elementary common sense, they cling to the Rambouillet formula that Kosovo remains part of Yugoslavia even after the war.
Do they really think it can? Do they really believe that having uprooted more than a million Kosovo Albanians, and sent 500,000 of them fleeing in terror, Mr Milosevic will meekly admit the folly of his ways and let them back in? These are the questions public opinion is asking ever more loudly.Reuse content