IT'S NOT FAIR what's happening in and to Sri Lanka, the small Indian Ocean democracy (formerly Ceylon) now fighting for its national life against a secession movement mounted by a terrorist sliver of its 18 percent Tamil minority. The problem is India. From close by on the mainland, its 50 million Tamils encourage and support not simply the welfare of the 2 million Sri Lankan Tamils but also the breakaway group among them. Ostensibly to provide ''humanitarian'' relief to Tamil civilians caught up in a Sri Lankan counteroffensive, the Indian air force last week dropped food and medicine in rebel-held areas. It was a rude intrusion that left some Sri Lankans fearing that a full-scale Indian effort to ''liberate'' Tamils could yet come.

You would think that Indian Prime Minister Rajiv Gandhi, who knows from the Sikhs what it means for a democracy to confront terrorism, would think carefully before offering to civilians relief that is bound to be exploited by terrorists. It is not that the Sri Lankan government any more fails to see the urgency of treating the fair grievances of the Tamil minority.

The Sri Lankan cause, however, appears to have a broad appeal in India, and Mr. Gandhi has been going through a patch of political adversity in which he may have seen the uses of playing to it. The practical result is likely to be to diminish the hearing that India can expect, in both official and rebel quarters, for its calls for a return to autonomy negotiations. Sri Lanka's terrorists may now be looking for other ways to apply the great weight of India against the beset forces of the island's government.

Thoughtful Indians are sensitive to the charge that they ''bully'' Sri Lanka -- and, by extension, that they act cavalierly toward other smaller neighbors, including Pakistan. But as it happens, India has recently been part of a buildup of tensions with its one neighbor that it cannot dominate -- China. In the same Himalayan region and over the same unsettled border, the two competing Asian giants conceivably may even have come near a renewal of their war of 1962. That was the event that pointed India, seeking strategic compensation, toward a treaty with the Soviet Union and detonation of a nuclear bomb.

It's not at all clear what has been going on in the Himalayas this time around. But it is big-league stuff, and, compared with it, dabbling in Tamil separatism on Sri Lanka is trivial, something unworthy of a major regional power.