A vast hodgepodge, a veritable smorgasbord of measures to "strengthen peace," was laid out by Leonid Brezhnev in his speech to the 26th Party Congress in Moscow last week. His purpose, massive evidence suggests, was to slow down American preparedness and split the United States from its allies.

Still, American opinion being what it is and the allies being what they are, the Reagan administration must make a positive response to at least one of the Russian gambits. The least noxious -- especially from the viewpoint of the allies -- centers around a French plan for a European Disarmament Conference.

This smorgasbord of proposals dished up by Brezhnev included practically every initiative ever put forward before by anybody. The Soviet leader smiled on a scheme, once advanced by West Germany, for a freeze on medium-range nuclear weapons in Europe. He revived an idea of his own for neutralizing the Persian Gulf. He tied that to a notion, backed in various ways by Pakistan, Iran, France and Btitain, for dealing with the Afghanistan problem. He indicated readiness to renegotiate the SALT treaty with the United States, and called for contacts -- though not, as widely reported, a summit meeting -- at all levels. He even had a kind word for the "changes . . . now taking place in China's domestic politics."

But the velvet glove ill concealed the iron fist. The bulk of the speech extolled a Five Year Plan, which continues inordinate emphasis on military spending and perpetuates the preponderance of heavy industry off which the military feeds. With respect to Afghanistan, Brezhnev made it seem that all that had happened was "limited" Soviet "assistance" called in by popular demand. Events in Poland were described as "a threat to the foundations of the socialist state."

A call for "deeds not words" would ideally be the right American response. But a sence of guilt about Vietnam still haunts American opinion. Many "enlightened" Americans share the view of the Carter administration that the interest in arms control transcends such mere bagatelles as Soviet aggression in Afghanistan, and repression in Eastern Europe, and adventurism in Africa.

The most prominent American allies, moreover, have interests, rooted in domestic politics, in doing certain kinds of business with Russia. West Germany drives a brisk trade east of the Elbe, and there is broad public support for Chancellor Helmut Schmidt's efforts to foster more rapport with the kith and kin in East Germany. French Gaullists and French Communists -- both critical to President Valery Giscard d'Estaing's hopes for reelection in May -- like to see their country playing the role of middleman between the super powers. The British, keen to make their way as Good Europeans, fancy a special role as broker between the United States and the Continent.

The Reagan administration cannot defy these feelings, however ill-founded. Going against public opinion in this country and the interests of the allies can only weaken the drive to stiffen American defenses and enhance U.S. influence. To give a harsh answer to the Brezhnev speech would be to play Moscow's game.

But one proposal in the Brezhnev grab bag can, temporarily at least, be explored without great risk. The French plan for a European Disarmament Conference would not in any way compromise the projected American military buildup. In supporting it, the United States associates itself with the only ally that, for now anyway, shares this country's willingness to apply muscle against moves by Russia or its proxies in Africa and Asia.

Moreover, the Russians have already given ground on the proposal. Originally the French called for confidence-building measures, including advance notification of troop movements in Western Europe, Eastern Europe and European Russia from the Urals west. The Russians refused to include their own territory. In his speech last week, Brezhnev said the proposal could apply to Russia west of the urals. He hinted that some American territory should also be included. But having elicited one concession by standing firm, this country and its allies are in good posture to pocket the gain and hold tight to the original proposal.

Not forever, though. In time pressures will build for matching concessions. Or for a U.S. move to the summit. Or for exploration of what can be made to look like a Soviet offer to leave Afghanistan in return for a neutral Persian Gulf. Eventually, furthermore, a France backed without limit by Washington will produce a Gaullist Europe dominated by national feelings. A nationalist West Germany, in those conditions, might well elect to turn its back on the Atlantic, the better to cut a deal with Russia.

So East-West relations are now on a slippery slope. If the Reagan administration is to marshal American opinion and avoid a split with the allies, it has to get on with the business of building American defenses, and drawing West Germany, Britain and France into a joint global strategy for dealing with Russia.