THE SOVIET UNION is making an unprecedented investment in once-nonaligned Afghanistan, where a civil war is raging between an unpopular Soviet-leading regime pinned in Kabul and Islamic tribesmen roaming in the countryside. To its 5,000-plus advisers, the Kremlin has now added some 1,500 "combat-equipped" troops in an effort evidently intended to end Afghanistan's traditional buffer status and make of it a Soviet client state. This Soviet initiative is a gratuitous power grab, making Moscow the fighting enemy of Afghan nationalism and of the Islamic movement as well. It represents a gross violation of the regional status quo. It is a direct threat to the stability of neighboring Iran and Pakistan, and it has alarmed many other countries.

For some time as the battle mounted in Afghanistan, the United States pretty much stood by, watching others help out the embattled Islamic elements but not encouraging them directly itself. With the evidence that the Kremlin is upping its ante, however, the administration has taken a more active public stance. It has moved from regarding the Afghan conflict as essentially local in nature and implication to treating it as part of a regional pattern. It is now trying to draw international attention to the fact and scope of Soviet intervention, emphasizing its anti-Islamic content and contrasting it with the United States' own benevolent attitudes toward Islam -- notwithstanding Iran. This is a piece with the administration's broad-gauge "post-shah" attempt to come to terms with Islam as a phenomenon in its own right, not merely as one aspect of a particular country or region.

The change of focus makes sense, but it is incomplete. American diplomacy is still not fully engaged. Why doesn't the United States take Soviet intervention in Afghanistan to the United Nations? Oh, no, some might say, the Russians are "helping us" by voting against hostage-taking in Iran. Not so: the Russians, by those votes, are protecting their own diplomats. But, others might say, it would hurt the ratification of SALT. Again not so: what hurts SALT most is the impression many people have that, for the sake OF SALT, Jimmy Carter will tiptoe past almost any Soviet power play. If the administration finds something unacceptable in Soviet policy in Afghanistan, it should take its protests out of the code of diplomatic signals and put the issue in a forum where the American voice will be clearly heard. Otherwise, everybody, including the Russians, may conclude the Mr. Carter doesn't really care.