WHEN senior military commanders start popping up in public asking for clear instructions on what to do in Bosnia, it is tempting to believe that they are really signalling their reluctance to become involved at all. This is not quite true. Field Marshal Sir Richard Vincent, chairman of Nato's Military Committee, is as aware as anyone that the credibility of Nato and the United Nations is at stake, and that the entire case for maintaining an expensive military establishment in the post-Communist world will be undermined if the armed forces can do nothing about such an obvious threat to European security as the war in Bosnia.

What is driving the military establishment of Nato to distraction is the vacillation of politicians on both sides of the Atlantic who refuse to confront reality and keep looking for cheap, risk-free options. This is not because the soldiers are eager to become involved in Bosnia. Far from it. They see the dangers clearly, and would like to avoid them. Military romanticism died in the First World War, and commanders are now commendably anxious to avoid casualties. But if they go to war, they need backing for every level of escalation that may be necessary to win. So far, the politicians have ducked that necessity, at least in part because of a commendable reluctance to send young men to die in foreign wars.

The tragedy of the Bosnian situation is that it is too late for cheap options. There are now broadly four tasks the military might undertake. The most limited would be to weaken the Bosnian Serbs by bombing their fuel and ammunition depots and cutting their links with Serbia. This might force them to sue for peace in due course but, in the meantime, it would probably intensify the fighting. UN forces would become belligerents. British and other contingents would be exposed to attack, including bombardment of their bases and guerrilla war against their supply lines. Humanitarian work would cease. Some Serbs, imbued with a fanatic belief in Serbian mythology, might 'do a Waco', mounting suicide missions designed to wreak as much death and destruction as possible before defeat.

A further limited task would follow political acceptance of the carve-up of Bosnia and require the military to protect a compact Muslim state, leaving the rest to be divided between Serbia and Croatia. This is favoured by Paddy Ashdown. It has the merit of realism, but would concede virtual victory to the Serbs. A more ambitious aim would be to enforce the Vance-Owen plan. Politically, this would be the easiest to defend, but would require complex military operations to police scattered enclaves and mixed communities.

The most ambitious mission would be to impose a UN protectorate on Bosnia by deploying a very large number of troops with iron determination over a long period. Since all the lesser options may escalate towards this one, it has to be faced, and perhaps ought to be chosen from the outset, expensive and dangerous though it is.

All options must be measured against the consequences of doing nothing or failing at a lower level of endeavour. Europe's most dangerous moments have always come with the collapse of empires. We are now at such a moment. As Mr Ashdown said on the radio yesterday, if Europe lacks the will to project its power to secure peace around its borders, we are in for an extremely painful decade.