PARIS. — There are four possible justifications for intervention in the Yugoslav crisis. The first is humanitarian: to defend convoys and air missions to feed people in besieged cities, evacuate casualties and civilians, rescue prisoners in concentration camps, etc.
The second is to seize and punish war criminals. This, and the preceding, are supposed to be the subject of resolutions to be put before the U.N. Security Council.
The third is to defend or restore the international frontiers between Serbia, Croatia and Bosnia-Herzegovina, which have been recognized by the European Community and the United States but have been overrun, chiefly by the Serbs, to create a ''Greater Serbia'' uniting all of the ex-Yugoslav regions in which substantial Serbian populations exist.
The fourth is to halt the quasi-genocidal Serbian (and Croatian) campaigns of ''ethnic cleansing'' of the territories they have seized.
The first objective tends to contradict the third and fourth. Helping the besieged without ending the siege is merely to help the victims of aggression survive today to die tomorrow. Evacuating civilians and prisoners from territory meant by the Serbs for ethnic cleansing is to do the Serbs' work for them at international expense. In circumstances such as these, the second U.N. objective -- punishing war criminals -- is mere rhetoric and will never be achieved.
In short, the Western governments continue to search for ways to satisfy an outraged public opinion while avoiding the serious issue posed by this crisis. Once again, as at Munich in 1938, the chief concern of Paris and London -- and today Washington -- is not to prevent or reverse aggression, but to find a face-saving way to avoid doing so. We still refuse to deliver arms to the Bosnians, and embargo others from doing so.
(One thinks of the British and French ministers in Prague awakening Czechoslovak President Edvard Benes from his sleep at 2 a.m. on September 22, 1938, when Hitler had delivered his ultimatum, to tell Benes that if war broke out the Western powers ''would hold the Czechs responsible for any catastrophe which followed.'')
Behind what Serbia is doing is the superficially reasonable, but in practice pernicious, theory that every ''nation'' should have its own state. This was a product of the 19th-century breakdown of the dynastic system. Monarchies in old Europe ruled over different ''nations,'' peoples and religions. Political unrest or rebellion might be generated by misrule, but not because the rulers belonged to a different nation or race: that was taken for granted as the way things always had been.
After the French revolution and Napoleon, people became convinced that they should fight not only for political and religious liberty but also against being governed by foreigners. The great 19th-century liberal historian, Lord Acton, wrote that ''protest against the domination of race over race . . . grew into a condemnation of every state that included different races, and finally became the complete and consistent theory, that the state and the nation must be co-extensive.'' He added that this was ''a retrograde step in history.''
As a theory, this was defensible. In practice, it has produced war, terrorism and what we have now learned to call ''ethnic cleansing'' -- a Serbian contribution to the political vocabulary we would have been better without.
It does so because, in Europe -- the main place where this theory has been applied -- the ''nation'' (what Acton referred to as the ''race'') has virtually no historical connection with the state. Britain, France, Germany, Spain, etc., all are combinations of many different ethnic stocks. The Prussians, who created modern Germany, are not Germans at all, but Balts, like the Lithuanians and Latvians. The Normans of France, who conquered England in 1066, were of Viking origin. The Anglo-Saxons are German tribes who settled Britain after the Romans left. Etc.
On the other hand, Serbians, Croatians and Bosnians are all the same people, speaking the same language, who profess different religions and have had distinct historical experiences, producing lasting hostility among them. There is no ''ethnic'' conflict in Yugoslavia. The war is one of religions and histories.
The ancient pattern of migrations from central Asia to Europe left each successive wave of peoples mingled with those who went before, or settled in overlapping areas. This mixture had relatively undramatic consequences until the modern idea of the nation spread to the backward regions of Eastern and Balkan Europe, then emerging from Hapsburg or Ottoman domination. Each individual ''race'' or nation there became convinced that it should have its own state and government, its own army, its own exclusive frontiers -- which, naturally, it defined in as large and ambitious of terms as possible. In 1918 the United States lent its weight to this idea that every ''nation'' should have its own state.
In theory, one can accept an argument that if the Serbs want their own state, or the Croatians, they should have it. The practical objection is that they can only have it at the expense of others. If those two were capable of negotiating a new frontier between them, and a peaceful transfer of populations that would permit each the primitive satisfactions of living exclusively with members of its own ''race,'' the international community would have little reason to object.
What is unacceptable is that they expand by aggressive war, conquest and ''ethnic'' -- which in their case means religious -- purge and murder. That would seem a simple enough principle for the Western powers to defend. Unfortunately, Washington, London and Paris seem unwilling to defend it, or even to admit that this is indeed the principle at stake.
William Pfaff is a syndicated columnist.