European leaders yesterday began to grapple with the prospect of either sending ground troops into the Kosovo war zone - with the near certainty of serious casualties - or seeing once again the collapse of Western Europe's pretension that it can mount a serious military operation independent of the Americans.
The Kosovo operation has always been conceived as a European-led mission, with the continent providing most of the planned 28,000 troops, while the US delivers the air power, and logistical and intelligence support.
That was the bluff President Milosevic of Yugoslavia began to call at the weekend, sending Serb armour and heavy artillery and reinforcements into the province of Kosovo in the belief that the prospect of a fight without US ground support would make the Europeans back down.
NATO strategy is now pinned on the faltering hope that air power alone will be enough to bring Mr Milosevic to heel - or at last to give him the excuse to tell the Serbian people that in the face of overwhelming NATO air strikes, they must reluctantly accept the Western-brokered autonomy agreement for the province where militants of the ethnic Albanian majority are fighting for independence.
The air power is all in place, from the cruise missiles meant to destroy the Serbian anti-aircraft missile and military communications systems to the F-117 and B-2 "stealth" bombers that could then target the Serbian tactical HQs.
Yugoslavia, for its part, has "a robust, highly integrated, well equipped air defence system - taking this on with air power will not be easy. There is a distinct possibility that we will lose aircraft," Gen Michael Ryan, commander of the US Air Force, recently cautioned in an appearance at the Senate Armed Services Committee in Washington.
Once the air defences, tactical centres and communications links were struck down, the remainder of the 300-strong NATO air armada could take aim at tank parks and bridges, artillery batteries and refuelling centres.
By all military logic, the Serb generals should then know that the game is up, but the Serbs have shown themselves adept at carrying on domestic repression while playing on divisions among foreign powers to weaken outside intervention.
NATO's Gen Klaus Naumann of Germany warned last October that air strikes alone were not a guaranteed solution: "Once war starts and the other side continues to resist, one must be prepared to go all the way, or hold back."
The Serbs are capitalising on the reluctance of the US to commit ground troops and the nervous hesitations of the European NATO allies at going into combat without the American presence.
NATO's supreme commander, Gen Wesley Clark, said at the weekend that the Yugoslav troop build-up in Kosovo signified "a vast and violent onslaught" to come against the ethnic Albanian Kosovo Liberation Army - and probably against Kosovo's civilians as well.
That is the moment of truth which NATO's European leaders now fear: a televised humanitarian disaster, which goes on despite air raids, forcing them to choose between sending in ground troops or backing down.
Gen Clark insists that a NATO strike would "devastate" the Yugoslav forces and the paramilitary security police. But the 1990-91 war against Iraq showed the limits of aerial bombing alone.
The grim fact is that NATO could probably not mount much of a ground operation. The alliance has 10,000 troops in Macedonia, but to fight its way in, under the Gulf War doctrine of overwhelming superiority, NATO would need at least 100,000 troops with heavy armoured support against Yugoslavia's army of 115,000 men, 60,000 well armed paramilitary police and over 1,000 tanks.
NATO does not have such resources in place and could probably not deploy a force of this size without the presence of US armoured divisions. Having been promised by the White House that Kosovo would be a European mission, the US Congress would take some persuading to risk Americans returning in body bags.
This challenge comes before European leaders have managed to finalise a pending agreement on a European Union military structure, attached to but distinct from Nato.
Under plans put forward by the German government, in response to an Anglo-French agreement at the St Malo summit in December, this would include the chiefs of staff of EU member-states' armies and top defence ministry officials, meeting regularly in Brussels as the EU military committee.
However, just as the EU's first serious attempt at big-time diplomacy collapsed over Bosnia five years ago, so Kosovo looks to be a proving ground that could test to the limit the embyronic "European defence identity" now being drafted.