AMERICA'S Joint Chiefs of Staff, the top brass who run the armed services, think their man in Europe has gone native. By his behind-the-scenes agitation for the release of ground troops to end the Kosovo crisis, General Wesley Clark is going against the Pentagon grain.

Nato's Supreme Commander advocated a land war, or at least the credible threat of one, from the start. Backed by Britain's belligerent Tony Blair, a hawk with no talons in the shape of available UK military manpower, he told alliance political leaders that air power alone could not stop ethnic cleansing.

That he has been proven right is irrelevant. The numbers game has moved on in the five weeks of inconclusive aerial bombardment. The US high command has set its collective face against becoming embroiled in an open-ended Balkans conflict with the potential for heavy casualties.

In the least damaging scenario drawn up by military planners, the alliance would have to spend up to $10bn, commit up to 100,000 men, and absorb up to 2000 dead and wounded just to smash existing Serb forces inside Kosovo. It would also take three months to organise and implement.

The worst case envisages 10,000 American casualties and perhaps the same again for the joint European contingents who would have to fight alongside them to overrun Serbia itself. That level of campaign would demand 200,000 allied troops and a war chest of $25bn. It would not be possible to build up a sufficient logistics base for it before next winter.

These two examples are those already on paper. But when the planners have gone away, as instructed at last week's Nato 50th anniversary summit, and returned with upgraded options, every figure is likely to be inflated to Vietnam-level numbers.

The fudge was Nato's sole means of showing a united front while still staving off the evil hour when a decision has to be made on whether to send tanks and infantry over the Kosovo border and into harm's way. Bodybags are not vote-winners. And war is ruinously expensive.

The tacit understanding was that the exercise would take some weeks to complete, and that the logistical difficulties and numbers of fighting men required in its conclusions would stifle knee-jerk calls for a land campaign.

The military analysts conducting the number-crunching have not even been instructed to draw up plans for a mass forward deployment to Macedonia, Albania, or Hungary, nor to produce a timetable for putting the vital stockpiles of fuel, ammunition, and food in place to support such an effort.

In the existing ''worst-case'' scenario, that of a full-scale invasion of Serbia and the nightmare of a street-by-street, house-by-fortified-house battle for Belgrade, it would take between four and six months to put everything in place before the bugler even blew the charge.

That would mean an October or November kick-off in some of the most hostile territory in Europe and a winter campaign in terrain known to Serb and Albanian alike as ''those accursed mountains'', 6000ft ranges with steep-sided valleys and few negotiable passes.

The bad weather will have come in again by late September, when the first snows traditionally begin to fly on the heights, largely wiping out Nato's so far overrated air superiority.

Serb troops would have to be winkled out of bunkers at bayonet-point, always a bloody business for all concerned.

It is not a prospect which fills with joy those with experience of being at the sharp end. The Pentagon, the only command with enough manpower to make it happen anyway, will resist to the last round of congressional debate and conduct a fighting rearguard action against politicians fired with more misplaced martial spirit than common sense.

Three-quarters of the bombs dropped so far in the air campaign have fallen on fixed targets: barracks long ago evacuated, Milosevic's disused ''command centre'' in Belgrade, bridges over the Danube, and two broadcasting studios. Many have been hit repeatedly, bouncing rubble but inflicting no casualties on the Serb military.

Serb frontline strength in Kosovo has increased since Nato strikes began on March 24. More than half the ethnic Albanian population has already gone to become someone else's problem. Controlling and directing the flow of refugees has become a new strategic weapon, a precision tool for disrupting the economies of surrounding states and Nato's military preparations.

The political guidance of the war so far has been inept on both sides of the Atlantic.

Nato, and its Pentagon puppet-masters, will now try to ensure that the folly of the air campaign is not compounded by a costly and ill-conceived ground war.

Blast alert at base

A Nato base in Macedonia was put on extra security alert yesterday following two large explosions outside the compound in the early hours. The blasts occurred as two cars passed in front of the base near Kumanovo, 18 miles north-east of the capital Skopje, according to spokesman Eric Mognot.

Nato officials believe they may have been caused by hand grenades or other similar devices. No damage or injuries were reported.