THE RESURGENCE of violence between Vietnam and Cambodia will have come as no surprise to those who recall the years, if not centuries, of racial and national animosity between these very different Indochinese peoples. Indeed, even before Communist forces defeated American-supported governments in the two places in 1975, there were signs that their common political ideology would not dilute their traditional hostility. On the contrary, the nationalism that each new regime had honed on American "imperialism" was there, ready and sharp, to be turned against the other.

Frontier skirmishes, along a French-drawn land border and on offshore islands, broke out promptly. Efforts to ease the dispute by diplomacy failed. Last month the Vietnamese charged - credibly, in light of past Cambodian massacres - that Cambodia had committed atrocities in raids in Vietnamese soil. Hanoi, which has perhaps the fourth largest army in the world, then invaded Combodia. The Cambodians charged - credibly, in light of past Vietnamese muscling - that Vietnam wants to annex the "Parrot's Break" salient of Cambodia. Vietnam is short on food, and the Parrot's Break is fertile, it is noted. In the classic invader's style, Vietnam, after having bitten off a good hunk of its neighbor's land, appealed for talks. Cambodia wants Vietnam to withdraw first. There, for the moment, matters rest.

It would take a very mean-spirited observer, not to say an irresponsible one, to urge on these two war-weary Communist regimes in a war against each other. But that is not to say that the United States, with its own particular history in Indochina and with no diplomatic ties with either country, is in much of a position to offer advice, warnings, good offices or what have you. China, which enjoys high standing in Cambodia, and Russia, which enjoys it in Vietnam, may both be better situated to mediate, if their own rivalry permits them to. More likely Vietnam and Cambodia will have to come to terms themselves. The Communist governments inherited power in their countries, and history, too.