Access to the roads and railways of Slovakia, completing a surface route from Germany through the new Nato members - the Czech Republic and Hungary - to Yugoslavia's border, in theory opens the way to an assault on Belgrade, just 100 miles to the south.
It is the option that beguiles the armchair generals: easy terrain, straightforward supply lines, and a chance to strike at the head of the Serb beast, rather than the myriad limbs entrenched in Kosovo proper. But, in political terms, it is scarcely imaginable.
Going in from the north means a land force - 200,000 men according to a Pentagon guesstimate last year - three times larger than that needed to clear Kosovo alone. It means going for Belgrade, which means conquering not just Kosovo but all Serbia, and in the process destabilising Serbia's northern Vojvodina province with its large ethnic Hungarian population.
And, assuming it succeeds, this sort of invasion means a de facto Nato takeover of Yugoslavia, and the same problem that deterred the Gulf War allies in 1991 from marching on Baghdad to topple President Saddam Hussein - the prospect of turning Nato into an unpopular occupying power for years if not decades. If forces are sent to Hungary, they almost certainly would be only a feint to distract Slobodan Milosevic, forcing him to spread his defences more thinly.
If Kosovo and Kosovo alone is the target, then, all other things being equal, the simplest and most direct way in is via Macedonia to the south. Over 12,000 Nato troops are already there, originally intended to spearhead the 30,000-strong peace-keeping force included in the now defunct Rambouillet peace package.
Alas, all is not so simple. About 150,000 ethnic Albanian refugees from Kosovo already threaten to shatter Macedonia's precarious political balance between its own Albanian minority and a Slav majority sympathetic to President Milosevic. Macedonia therefore flatly refuses to serve as base for a ground offensive.
And were Nato to go ahead regardless, it would run head-on into the problem of Greece, a member of Nato but instinctively a supporter of Serbs against Albanians. Logically, men and materiel would be shipped to Thessaloniki and go north by road or rail into Macedonia. The only snag is, the socialist government of Costas Simitis might be toppled if it went along.
At best, therefore, Macedonia is likely to come into play only if Nato can create the "semi-permissive" environment for sending in ground troops - where Serb forces in Kosovo have been battered into virtual non-resistance, and an international force moves in, empowered by a United Nations resolution and, if needs be, without the assent of President Milosevic.
That sort of "blurred" invasion, requiring perhaps as few as 60,000 men, is the most likely endgame in Kosovo. But the softening-up can come only from the direction of Albania, to all intents and purposes now a Nato ally and protectorate, and a long-standing sanctuary of the Kosovo Liberation Army.
Already Nato spokesmen claim that despite Mr Milosevic's savageries, KLA guerrilla operations in Kosovo proper are growing - and, however much they deny it, almost certainly with the help of arms and training supplied by Nato. As momentum for a ground offensive develops, KLA incursions from Albania are likely to increase further, as a kind of Nato advance force.
Alas, however, any creeping land offensive will be hampered by the appalling terrain of the border country, where the few natural crossing points have already been identified and protected by the Yugoslav security forces. For that reason incursions may be short on heavy armour and long on air support.
The most potent weapons against Serb tanks and emplacements will be the 48 Apache helicopters requested by General Wesley Clark, Nato's supreme commander. The first are now in Albania, and could go into action within a week.
As the campaign progressed, more Nato forces would enter Kosovo - both heavily armed infantry from Albania and airborne troops, pre-positioned in Italy or aboard warships in the Adriatic. By then, battered by months of high-level bombing followed by blistering low-level attack, the Serbs would have had enough. That at least is Nato's hope. The events of the past four weeks suggest reality might be rather different.Reuse content