THE RUSSIANS are behaving cynically in the Horn of Africa and, by so doing, undercutting the interests of Africa, the United States and, finally, themselves. At first, eyeing a Red Sea base, they gorged Somalia on more than a billion dollars' worth of arms and thus emboldened the Somalis to sponsor the "liberation" of the Somali-people Ogaden region of Ethiopia. Many Africans were alarmed to see Moscow seeking a base and promoting an invasion. Armed attack, most Africans agree, is no way to redress the ethnic grievances arising from the arbitrariness of colonial borders.

Now the Russians are looking again for a Red Sea base and political influence - in Ethiopia. Again they have supplied more than a billion dollars in arms, plus hundreds of Soviet advisers and even more Cubans. Not only are Africans alarmed at this expansion of the foreign Communist presence; they and others also suspect that when, as expected, Communist-backed Ethiopian forces clear the Somalis from the Ogaden, the Ethiopians will keep going into Somalia.

Jimmy Carter voiced keen alarm about these developments yesterday. He all but conceded that his effort to encourage great-power restraint in Africa, in order to let Africans solve African problems, had not induced parallel Soviet restraint. The Somalis, he suggested, should rapidly open negotiations. Yet it seems that the Somalis would wish negotiations only if Ethiopia would consider self-determination for Somali tribesmen in the Ogaden. This the Ethiopians are unlikely to do, least of all when they are preparing a massive counteroffensive. Most members of the Organization of African Unity may deplore the Communist presence, but since that presence serves a government that so far is only trying to regain control over its own territory, the OAU is mute.

Through 1977 the Carter administration in effect gave Cuba the choice of tapering off its African activities or losing the opportunity to improve direct ties. Cuba chose Africa, and the improvement of Cuban-American relations is now in the freeze. But the Russians have far more at stake in their relationship with Washington. Ethiopia is heating up fast as a political issue in the United States. Increasingly, Mr. Carter will face demands to do something about this latest Soviet-Cuban power play. The Ethiopian question is bound to complicate his relations with Congress across the whole spectrum of detente issues, including those in which the Russians have a high interest. We don't like the idea of holding SALT hostage to Ethiopia, but the Russians should know that, as a practical matter, this is the way things are likely to move unless they apply restraint. Washington may be stuck on the Horn. Moscow is, too.