THE KREMLIN threw a curve ball at the Venice summit -- but it missed the strike zone. By announcing that "some [Soviet] army units" were "no longer necessary" in Afghanistan and would be withdrawn, the Soviet Union doubtless meant to deepen the tension between the United States, which has led the effort to make Moscow pay for its invasion, and the other summit countries, which have sometimes seemed ready to let Moscow off the hook. So phony and unlikely was the Soviet announcement, however -- Soviet troops seem nowhere near quelling the Afghan resistance -- that the summit seven had no problem composing a tough and skeptical response.

The seven called for any withdrawal to be confirmed, permanent and complete. Soviet officials will surely keep saying their "withdrawal" is a gesture worth repaying by the summit nations and others. But they should be held to providing solid evidence that they intend, as the Venice meeting put it, "for the Afghan people to be left free again to determine their own future." A return to Afghanistan's earlier neutrality and not a reversible dip in the force level is the right goal.

Largely because of the unity evoked by the troop announcement, Afghanistan became the main political subject at Venice.It provided welcome if temporary relief from the strains that have pulled at the alliance in recent months. Other useful business, however, was done:

Jimmy Carter and West German Chancellor Helmut Schmidt came to Venice quarreling over how to proceed with NATO's decision of last December. The alliance had agreed then to deploy new American missiles in Europe starting in 1983 and, meanwhile, to seek negotiations with Moscow over limiting the new Soviet missiles already being deployed against European targets. Mr. Carter evidently feared that if Mr. Schmidt, who is going to Moscow next week, pressed the negotiation-half there, the deployment-half might be lost. But at Venice, it seems, Mr. Schmidt was able to allay Mr. Carter's misgivings. Anyway, Mr. Carter pronounced himself reassured.

Politically speaking, the American-European-Japanese alliance is in a nervous state. Its members are uncertain how to deal with crisis beyond Europe and in Europe. The Europeans' greater relations with the Soviet Union is the basic source of the strain, and it is exacerbated by the political styles and requirements of the various leaders. None of the problems of the alliance is ripe for solution on a weekend hop, but the summits help remind the governments and their publics of their common bonds. As Venice showed, it can be a useful reminder.