THE PRESIDENT gave an awfully good speech yesterday. He was well prepared, forceful and he made a lot of sense. Serious people in this country, in Europe and in the Soviet Union ought to study his message.

What the president did was to put the United States into a negotiating position on the critical issue of the military balance in Europe and, by extension, in East-West relations overall. This is more than a public relations stunt, though an appeal to public opinion is vital to the diplomacy practiced by democratic countries. It is a serious effort to stabilize relations with the Soviet Union.

In content, Mr. Reagan's speech followed closely the NATO deploy-and-negotiate decision of 1979. Previously, the United States had held off approaching the table lest the spectacle of negotiation undercut an ambivalent Europe's support for deployment of new missiles to match the new Soviet SS20s. But it was finally decided to start talks promptly in an attempt to limit further public unraveling. Either way, European public opinion would hardly have stayed still. With talks begun, however, the United States can fairly say it is abiding faithfully by the terms of the NATO deal.

In the talks due to start Nov. 30 the president said he will offer to halt the new NATO deployment if the Soviets dismantle the daily growing force of new SS20s trained on Europe, and the older missiles for which the 20s are a replacement. Soviet President Leonid Brezhnev told a West German audience recently that that was not good enough. American "forward-based systems" (airplanes and sub-launched missiles) of intermediate range and the British and French independent nuclear deterrents must also be on the table, he declared. That's what the negotiation will be about.

How will it come out? That will depend in the first instance on the temper of the Europeans. Each great power is playing to them. The Soviets, as they observe the European "peace movement," may be sorely tempted to play to it in the hopes that eventually NATO will have to reduce or call off the new deployments without Moscow's having to make any concessions in return. No one can say confidently that this will not happen. The Europeans, however, have to ponder what then might result.

Ronald Reagan has just made a genuine and unthreatening commitment to the Atlantic Alliance. He has laid out a basis on which American support for European security can be sustained indefinitely, and he has put forward a respectable position from which to launch a serious negotiation. It is not for any American, however, to define Europe's concept of its own security. Europeans must decide for themselves whether it is better to respond or not-- on the double track on which they insisted--to the new Soviet missile force and to the political purpose represented by that force. In brief, Mr. Reagan has returned the question of Europe's security to the only place where it can finally be answered: Europe.