"The whole operation flies in the face of military common sense" was a particularly trenchant version expressed last week. "Beyond belief" was another. Even some pilots daily flying missions against Yugoslavia are said to be asking "What exactly is this achieving?".
Arguments for and against the deployment of ground troops have been well rehearsed by defence experts and high-profile generals over the past two weeks. But those of more lowly status, who for reasons of employment or service loyalty are unwilling to speak openly, maintain that this issue still goes to the heart of the problem. Their training, and the whole of modern military history, tells them that an air campaign on its own will not work.
"Air power is an oxymoron," said one. "You gain air superiority in order that your ground troops can do things. Air superiority on its own is worthless." Even given that air power is the option Nato has taken for political reasons, they say, the way the operation has been prosecuted has virtually guaranteed its failure. And it is a failure: judged on Nato's own terms when it entered the conflict, it has already lost. However much its commanders "ramp up" the scale of air strikes, however much they suck in more fearsome "assets", that conclusion is inescapable.
On the morning after the first bombs and missiles fell, George Robertson, the Secretary of State for Defence, stated: "The military objective of these operations is clear cut. It is to avert an impending humanitarian catastrophe by disrupting the violent attacks currently being carried out by the Yugoslav security forces against the Kosovar Albanians, and to limit their ability to conduct such repression in the future."
The most recent British figures show that 508,000 refugees have now left Kosovo - 439,500 since the offensive began. Other estimates put the total of displaced Kosovar Albanians at 1.1 million - from a population of nearly 2 million a year ago. What, then, has been averted?
"The first principle of war is selection and maintenance of the aim," said one military man last week. "On both counts we have got this terribly wrong."
The result, others say, is a Third World country running strategic rings around the most powerful military alliance the world has seen. And the best that official spokesmen can manage are frequent expressions of surprise over circumstances that could and should have been foreseen.
Yugoslavia, a former Eastern bloc country, is a perfect country for Nato to fight its first war against. It is against just such an enemy that the alliance has trained since its inception. Yugoslav weapons systems are all familiar, and the in-depth knowledge of its famed "integrated air-defence system" will have been further enhanced when Hungary and Czechoslovakia, with virtually identical systems, joined Nato.
Eight days of problems with the weather, where attacks were largely limited to cruise missiles, attracts particular derision. "Nato has been studying the climate of the former Eastern bloc for 50 years, so it should have known what to expect," said one former officer.
In that case, why weren't the aircraft with all-weather precision capability, such as the US B1B Lancer bomber, not deployed from the start? Given that a lot of the targets were against isolated military installations, such as ammunition dumps, why weren't aircraft such as the RAF Tornadoes, with the ability to fly blind and drop "dumb bombs" accurately, used when the laser-guided bombs could not function?
The much-vaunted US Apache helicopters have still not arrived. But, given that ground troops in Kosovo were always going to be a target, why not?
The fact that more and more aircraft have had to be summoned betrays a miscalculation about what was needed in the first place. It also meant that the campaign got off to a slow start, and vital advantage was lost. About 2,000 missions were flown during the first 10 days of Operation Allied Force; in the Gulf war that was the number on the first night of the air campaign.
And since every conflict in the Balkans has created a refugee problem, plans and supplies should have been in place before the operation began, so that Nato did not have to divert resources away from the air campaign for its humanitarian effort. In Bosnia, said one source, a brutal lesson had been learnt: "The only way to stop this kind of thing is to kill the people who are doing the killing."
Given that the ever-intensified effort against Serbian ground troops is still yielding limited results, something more radical was needed from the start, and is now urgently required. Again, the answer is there, say some, and has been practised for years - a huge airborne and air-mobile assault on an airport, such as Pristina. After that troops and equipment for a land operation could be flown in, accompanied by an armoured thrust from Albania and Macedonia. This would leapfrog the problem of mounting an initial land invasion through mountain passes, and air superiority should enable it to be carried out with minimised risks. Losses there will be, supporters of the plan say, but the wish to fight a "PC" war with no allied casualties has so far brought Nato to the brink of humiliation.
It may sound far-fetched, but just such a capability has been behind much Nato thinking and allocation of resources over the past decade. Only last summer Nato dropped 1,700 troops, led by elements of 2 Parachute Regiment, into the mountains of Albania for a five-day exercise. Airborne forces from the US, Italy, Germany and Greece were also involved and 10 other countries sent observers to the operation, codenamed "Co-operative Assembly 1998". A Ministry of Defence spokesman said at the time: "It is aimed at demonstrating to the Serbs that a substantial force can be put into the area at very short notice and in strength."
This particular threat has so far not been carried out. But unless the ever-growing Nato air armada can score a significant success soon, it may be a plan that has to be dusted off. Elements of the British Army, in particular the Parachute Regiment and other air-mobile forces, are said to have been getting increasingly restless over their lack of involvement.
It would require Nato commanders, and politicians, to bite a very big bullet if the "Paras" and allied counterparts are to be given their head, but if Nato's credibility is to be rescued, they may ultimately have little choice.