IT IS A STRIKING tribute to "normalization" that a trip to Peking by Defense Secretary Harold Brown could have been scheduled as early as the start of the second year of full diplomatic relations between the United States and China. For such a trip could have only one purpose -- to emphasize and explore different kinds of security collaboration, surely the most sensitive sector in matters lying between Washington and Peking and the one most fraught with implications for the relations of each with Moscow. Inevitably, the Kremlin's aggression against Afghanistan sharpened the focus, making more urgent and publicly supportable the need for tightening Sino-American security links. This seems to be just what Mr. Brown did in his talks in Peking.

The fact of the China connection is for Washington its greatest strategic benefit: just by existing, the connection halves the number of our nuclear-armed foes and, if you take the West as a unit, doubles the number facing the Russians. Anything further we get from China, whether by way of support for pro-American forces in the world or reduced support for anti-American forces, is gravy. China, meanwhile, has sought three things from the United States, and from the West as a whole: modern military hardware and technology; a tougher American stand on Moscow; and cooperation, or at least understanding, on particular trouble spots. Increasingly since normalization -- not just in the last two weeks -- the United States has felt it was in its interest to help out in these matters: to make China "strong and secure."

Before Afghanistan, a debate ground along inside the American government over whether the United States should use military cooperation with China as a level to pry Moscow toward political cooperation, even at the risk of souring Moscow on "detente." The Afghan affair, or rather the long cycle of events it seemed to culminate, did not end that argument, but it was a blow to those who had argued that the priority was to keep the line to Moscow clear. For any additional insecurity the Kremlin may feel as a result, it has itself to blame.

The Chinese have their own continuing doubts about American staying power and about the wisdom of heavy reliance on any foreign country. So they evidently are not eager, even now, for an out-and-out alliance with the United States. But Harold Brown did, it seems, take some substantial steps forward with them: in technology, in consultation on the Afghan situation and, perhaps, in broader strategic explorations. Presumably he did so without mortgaging this country's policy to whatever plans of its own Peking may have to teach another "lesson," by force, to Vietnam. His talks in Peking look like a success.