Paris — First you take the underground Metro to the north Paris stop Saint Georges. Then you walk down the hill and push open a drab door around the corner from a sidewalk cafe and opposite a car showroom.
You walk up a worn, winding staircase to the third floor (there is no elevator), and you find a single room on the left.
Inside is a square of green carpet, an ancient filing cabinet, a single desk, and one of the loneliest groups in all of France.
Its aim is to oppose nuclear weapons in Europe. Similar groups in Britain, the Netherlands, and West Germany have commanded international attention over the last two years. ''But in France we are very small,'' concedes tall, bearded activist Michel Leter from behind the desk.
But why? Why is France the weak link in the European peace protest chain - a chain already weakening as newly reelected conservative governments in Britain and West Germany prepare to accept United States cruise missiles to counter Soviet SS-20s?
Doesn't France have a Socialist government, a strong Communist Party, and a Gaullist history of political independence from NATO? Didn't a peace rally at Vincennes June 19 draw crowds estimated at between 80,000 (by the police) and several hundred thousand (by the organizers)?
Yes. But. . . .
The situation is very different here. ''It's so hard for us,'' says Sylvie Montrant, who is in charge of relations with other peace groups for the group Michel Leter works for, CODENE, the French Committee for Nuclear Disarmament in Europe.
An umbrella organization for 25 smaller groups, the committee claims a total membership of 25,000. But Miss Montrant puts the activist core at no more than 500.
In her apartment on the edge of Montmartre in Paris, she ticked off some of the main reasons. Agreeing with her in separate interviews were other analysts, including author Alfred Grosser, director of research for the National Political Science Foundation and professor at the Institute of Political Studies.
* There is a national consensus here for the de Gaulle-created nuclear force de frappe, or nuclear arsenal. Not only the Socialists led by President Francois Mitterrand, but also the Communists are behind it.
The key concept is French independence. The Communist Party has to support it or face the charge of being a tool of Moscow. President Mitterrand firmly believes in it. And according to the French public opinion poll SOFRES, the percentage of French people believing that the nuclear deterrent is ''positive'' for France has risen from 53 percent in December 1977 to 66 percent in April 1983.
''That's because the left switched to supporting the force de frappe after 1977,'' said Professor Grosser. ''Independence is the magic word. Without the force de frappe there'd be no reason for France not to be militarily integrated into NATO again. . . .''
* On medium-range cruise missiles coming to Europe, only the Communist Party is opposed.
President Mitterrand's Socialists accept the December 1979 two-track NATO decision to install cruise and Pershing II missiles in Western Europe unless agreement is reached with Moscow at the Geneva missile talks.
Mr. Mitterrand genuinely sees an imbalance in East-West nuclear forces - and his support of cruise and Pershing installation is reassuring to other NATO allies concerned over the presence of Communists in the French Cabinet.
To come out publicly in opposition to the missiles, then, is to be aligned in the public mind with the Communists. At a time when Lech Walesa is a national hero to old and young French people alike for his opposition to Warsaw and Moscow communist parties, not even CODENE is willing to attend any ''peace'' rally organized by the Communists of France.
That is why CODENE and the Socialist Party stayed away from the Vincennes rally, which was in fact a forum against cruise missiles that did not include balancing criticism of the Soviet SS-20s.
* There is no national opposition to civil nuclear power plants either, as there is in Britain, West Germany, and elsewhere. France is racing ahead with its nuclear program. By the end of 1981, according to the International Energy Agency, French plants were generating 22 gigawatts of power - second only to the Soviet Union.
* Just beneath the surface of French life, French people except the young retain an abiding concern about West Germany. People worry that economic recession will break up German politics as it did after World War I. This leads to two results:
1. On defense strategy, President Mitterrand takes a publicly pro-NATO, pro-US line partly to try to ensure that West Germany stays encased in the NATO framework.
2. It convinces most French people that, as Professor Grosser puts it, ''the lesson of 1938 is that France must never again be weak or disarmed in the face of a dictator.''
A Western diplomat who studies French internal affairs in Paris comments: ''I don't see any short-term future for the French peace movement. . . . It loses credibility because it is at once identified with the Communist Party. . . .
''The peace movement as a whole does better in Protestant northern European countries like Britain and the Netherlands. In southern Europe, in countries with large Communist parties, in France and Italy for example, the movement does worse.''
This leaves people like Sylvie Montrant and Michel Leter struggling uphill to influence French opinion.
Refusing to be associated with the Communist-dominated ''Mouvement pour la paix,'' CODENE plans a rally at Larzac in southern France for Aug. 6 and 7 to call for a freeze on new warheads and a new submarine for the force de frappe, and a march in Paris in October to coincide with a protest against cruise and Pershing deployment in Brussels.
''On the BBC from Britain, we hear people openly debating nuclear missiles and strategy,'' says Michel Leter with admiration. ''In France there just is no such debate. Britain has a debate with logic as well as emotion. Here, we have only emotion. . . .''
Miss Montrant is trying to enlarge CODENE by talking with trade unio s and with Protestant churches in France.
Meanwhile, Professor Grosser warns that one way to boost pacifism in France is for President Reagan to continue using strident anti-Soviet rhetoric.
''There is no contradiction between firmness and detente,'' the professor said, ''but President Reagan could help turn the French into pacifists by using language that is unacceptable to all sides. You have to be firm with Moscow but you have to negotiate with it. The Soviets are compulsively aggressive but reasonable at the same time. . .''