SRI LANKA, the island nation south of India that has had success in recent years practicing democracy and free enterprise, blew up the other day. The Sinhalese majority took out after the Tamil minority, killing hundreds, leaving tens of thousands homeless, inflicting economic damage in the hundreds of millions of dollars and causing the nation's prospects to shrivel almost overnight. Everybody knew--for centuries--there was bad blood. Nobody seems to have anticipated an explosion on this scale. A country headed up, despite everything, is heading down.

In Sri Lanka (Ceylon until 1972), American missionaries from Boston educated the Tamils, who came from India in the early 19th century to work on the tea plantations. This was one factor contributing to the Hindu Tamils' advancement in a society otherwise dominated by Buddhist Sinhalese. (It also apparently accounts for the resolution in the Massachusetts legislature denouncing Tamil repression.) There has been trouble for a long time, although the country knew periods of relative communal harmony, too. In the latest episode, Tamil separatist guerrillas ambushed a patrol of Sinhalese soldiers. Riots broke out. The Sinhalese army seems, at best, to have stood by.

Everywhere in the world different ethnic groups find themselves sharing a single set of national boundaries. Why can they get along passably well in some times and places and not in others? It is easy enough for outsiders to urge the Sinhalese and the Tamils to put aside their differences and work together for their common good. But other currents are running. The president is well thought of but he has an inflamed constituency to answer to. Citing a danger of Tamil secession, he has closed out Tamil representation in Sri Lanka's elected parliament, a move whose likely effect will be to still the remaining moderate Tamil voices and leave the field to the guerrillas. The "Tamil Tigers," who have been helped by the PLO in the past, have India's Tamil province nearby for auxiliary sanctuary.

If living together is so hard, what about a separate state in the north for the Tamils? They have as good a claim to a nation of their own as most members of the United Nations. But as always it is a question of power, and in Sri Lanka the Sinhalese have the power. Do they also have the wisdom to see that the Tamil minority is treated in a way that justifies its retention within a unitary state?