Suppose the large Hispanic population in the American Southwest decided it was time to form a nation of its own. The ethnic and linguistic ties to Latin America are strong, and everyone knows that the Sun Belt is the spoils of war ceded to the United States by Mexico in the last century. If Hispanics wanted autonomy, would other Americans let them go and take Los Angeles, Phoenix and Houston with them?

Not a chance. The South tried that once before, and the lesson of the Civil War is that nobody secedes from the United States without a fight.

Yet what Americans find unthinkable at home, they are cheering in the Soviet Union. Many in the West are lauding the secessionist rumblings in the Soviet republics as a red-white-and-blue display of democracy. A spate of independence movements is unraveling what remains of the socialist monolith, and Americans couldn't be more pleased. Every nationalist declaration by the republics is greeted by a round of "I told you so" from the United States. The uprisings are belated justice -- a confirmation that the oppressive system was a mistake from the start.

It is not popular to call for moderation when ethnic pride and long-suppressed nationalism are sweeping among the people who are Soviets through no choice of their own. The international right to self-determination is sacrosanct. It has even more appeal when its target is a waning, totalitarian empire. But not every sign of failure in communism is necessarily a step toward democracy as practiced in the West.

There are politically and economically sophisticated societies such as the three Baltic states that have proved they can stand alone. Lithuania, Latvia and Estonia got along fine without Soviet help before Stalin swallowed them up during World War II. As they hammer out the terms of their independence from Moscow, it looks like they will be able to avert violence.

But the separatist agenda meanders through other republics where there is little experience with independence and probably little idea of what it means. They have an understandable zeal to break away. But too often it is accompanied by no political, economic or strategic logic. It is no wonder that Moscow is scrambling to get its nuclear warheads out of those hot spots.

Americans should be more alarmed than pleased when they see nationalist sentiments welling up among the Moslem republics. Cut free from the U.S.S.R. with no plan for self-governance, they are ripe for the picking by their fundamentalist neighbors in the Persian Gulf.

In some of the republics, the values of political pluralism and a free market sometimes have nothing to do with the separatism movement. Sometimes, all that is at the root is a grudge against the ethnic group next door.

Examples of the chaos that results from such nationalism abound within the Eastern Bloc, but the best illustration is Yugoslavia. The dizzying array of religious and ethnic groups are eternally at odds. The infighting obscures poverty and instability. All the Yugoslavs have gotten for their fractured ethnic pride is bloodshed.

Would Americans say that all the Eastern Bloc ethnic groups have the right to their own country? That argument cloaks separatism in the fancy garb of democracy, when its underpinnings are really more mundane.

Meanwhile, an entire region tilts toward chaos as everyone from Georgians to Ukrainians to Central Asians to Turks to Transylvanians to Albanians to Slovenians to Slovaks demands the recognition as distinct peoples that was denied them by Moscow.

Advocates of autonomy often correctly point to the fact that these strange bedfellows were fused together against their will by an evil empire. That history has long been the burden of emerging nations. But sometimes it is best to make something out of the present union rather than try to go back to the fractured past.