FRANCOIS MITTERRAND, the president of France, chose a juicy red strawberry from his breakfast bowl and with a delicate pivot of the wrist brought it up to his mouth just as he finished saying that the Palestinian problem was not something he had to, nor could, solve. Indeed, Mitterrand observed to an American journalist munching cornflakes beside him, it might just be a problem that no one could solve.

Approaching his sixth month in office, Mitterrand has settled in as a medium-sized man content to direct a medium-sized world power and to enjoy its limitations, rather than trying to follow through on the much grander geopolitical designs of his patrician predecessor, Valery Giscard d'Estaing. In a 90-minute conversation during his first American visit last week, the list of global problems that Mitterrand does not recognize as demanding his involvement is impressively long.

Indeed, he is intent on reshaping the French economy to fit France's size, to protect it from the giant economic forces he sees moving across the globe through multinational corporations and the disorderly trading patterns that have been traced by the oil, inflation and dollar shocks of the 1970s.

Mitterrand has been insistent on carving out for himself one niche on the international scene, and that is the unusual one of being a French socialist president who wholeheartedly supports America's nuclear rearmament program, including the controversial deployment of a new generation of U.S. nuclear missiles in Europe that will be able to strike into Russia. Despite the lingering image for many Americans of Charles de Gaulle's France as America's reluctant ally, Mitterrand has supported the nuclear program with more vigor than any other European leader. And he has been able to do it without provoking the kind of domestic opposition that is rending West Germany's ruling Social Democratic Party, shaking the Belgian and Dutch governments, splitting Britain's Labor Party and beginning to stir in Italy.

Ironically, Mitterrand's ability to do this is in large part a legacy inherited from de Gaulle, the titan of a ruler who scorned Mitterrand during his lifetime but who in many ways paved the way for him. De Gaulle made nuclear weapons and national independence synonymous terms for Frenchmen, including finally the French Communist Party that is happily voting for Mitterrand's defense budget. And the bruising Franco- American squabbles of the 1960s may have provided some valuable inoculation in France against the virulent but relatively unsophisticated brand of anti-Americanism that seems to be on the rise in Europe today.

But that does not explain Mitterrand's willingness to become a political pointman on nuclear weapons for the Reagan administration. The answer to that puzzle lies in part in the historical and literary personality of Francois Mitterrand, a complex and fascinating man, and also in his concept of the nature of American and Soviet rivalry in Europe today.

He came to Williamsburg last week on a formal mission, to join Ronald Reagan in celebrating the 200th anniversary of the defeat of the British by French and American armies at Yorktown. But in his speeches and toasts with Reagan, a press conference and the breakfast interview, he made clear that his true mission was to demonstrate to Americans that they have one reliable ally in a Europe plagued with political and economic divisions, and that is Mitterrand's socialist France. In return, he told Americans, he was keenly aware that the United States is France's indispensable ally against a Soviet strategy of threatening Europe.

The fashioning of a new French-American entente had begun during Giscard's seven-year term. But it was a largely covert affair, due to Giscard's feeling that public acknowledgement of the improving relations and any support by him of U.S. nuclear plans would endanger his ambition to become the middleman between Washington and Moscow. A reporter who has interviewed both in power quickly learns at breakfast with Mitterrand that the new president has abandoned that ambition, along with the haughty mannerisms and tone that helped defeat Giscard in his bid for reelection last May.

At the Elys,ee Palace in Paris, Giscard served scrambled eggs with truffles to visitors. In his last years, stories about Giscard's supposedly royally inspired refusal to have anyone seated in front of him at a meal or to let anyone, including visiting heads of state like Margaret Thatcher, be served before him circulated widely in France and helped the Socialists portray him in the campaign as a would-be monarch. The aloof, withdrawn Giscard defended himself poorly against these charges.

At Bassett Hall, a small plantation house restored by the Rockefeller family in colonial Williamsburg and turned over to the French Socialist leader for his stay, a waiter at first sets bowls of cornflakes topped with strawberries before each of the 10 place settings. Then, after a hurried conference with a Mitterrand aide, one bowl disappears and returns sans cornflakes and filled with strawberries. Everyone in the room now knows where the intensely nationalistic and cultured Mitterrand will be sitting.

He is at Williamsburg two weeks before his 65th birthday, and his frequent long walks in the forests around his country home at Latche in southwestern France have left him in apparent ruddy good health. Rotund without being portly, he seems slightly distracted with an interior monologue even at the most intense moments of the conversation, his thin, graceful fingers knitting and unknitting themselves as if he is straining to write down rather than say his words, a habit that may grow out of the eight polished though not profound books on modern France and politics he has written.

If Ronald Reagan is an actor who has reached his greatest role in taking on the American presidency and its extensive stagecraft, Francois Mitterrand is a writer who is composing his presidency, crafting a literary work to encompass the political realities and visions of Western Europe's oldest centralized state. This is perhaps one of the keys to understanding the oddity of Mitterrand's getting along so well not only with Reagan but also with Secretary of State Alexander Haig, who appears to create himself anew in each situation, sometimes disastrously and sometimes brilliantly, and who can presumably appreciate the existential nature of Mitterrand's task as writer-president.

Moreover, Mitterrand frames a good measure of his political perceptions against his finely developed sense of literature. His analysis of revolution in Central America, at the root of his extraordinarily strongly expressed differences with Reagan on El Salvador, has the sweeping romance of a novel by Gabriel Garc,ia Marquez or Carlos Fuentes. His Russians resemble Tolstoy's Russians. The American profile he carries around is less clearly expressed, but aides say he admires Henry Miller and William Styron among modern authors.

One aide who has known him for a long time thinks Mitterrand is condemned in his early days in office to fall back on what, and who, he already knows well. This aide expects changes as the president deepens his knowledge of the official files and conversations that were relentlessly kept away from him since de Gaulle took power. "You in America have no idea what it is like to be shut out, completely out, of the policy information network for 23 years, and how long it takes to go through it all."

When Mitterrand swept to an easy victory on May 10, many observers concluded that Al Haig had lost what he reportedly called in private "our Cubans." Giscard had shown himself willing to use French troops in Africa to shore up regimes friendly to the West, and had easily swallowed the administration's case on El Salvador. It was clear that Mitterrand would be much more difficult on the Third World; it was not clear if his well publicized suspicion of the Russians would be forcefully enough expressed to persuade the Reagan White House to overlook the Third World differences and Mitterrand's decision to name four Communist Party members to his cabinet and extend nationalization deeper into the industrial sector.

The initial results weigh heavily in Mitterrand's favor. Haig and Co.appear pleased with Mitterrand's strong anticommunism, which has been sharpened by his own successful battles to tame the authoritarian French Communist Party, and by what they take to be his anti- Sovietism. But in the discussion at Bassett Hall, which Mitterrand asked be paraphrased when rendered in English rather than quoted directly, his thoughts on the Soviet Union, West Germany's nuclear dilemma and the course of politics in Western Europe appear to still be taking shape and root as he digs through the dossiers kept away from him since 1958.

Giscard believed in Leonid Brezhnev's sincerity, and thus believed that peace was primarily a technical problem of finding the means to identify and codify the shared goals between the Russian leader and the American president of the moment. Thus he constantly explored ideas for peace conferences and kept up his personal contacts with Brezhnev, including the politically and diplomatically damaging trip to Warsaw in 1980.

The point of departure for Mitterrand is that the Russians do not want to fight a nuclear war. This generation of leadership remembers, as he does, World War II too graphically to risk such destruction. That makes it imperative, he believes, to negotiate an arms control agreement now, while Brezhnev and his peer group are in charge.

But he also believes that the Soviet Union has adopted une strategie de la menace, a strategy of threat based on gaining a nuclear superiority over the United States that would enable Moscow to demand whatever concessions it wants in and from Europe. By the calculations he has found in the strategic plans left for him by Giscard and supported largely by American information, he concludes that such a situation will exist by 1985.

Thus, he has applauded Reagan's decisons on the MX missile and B1 bomber, apparently without having looked closely at either. His enthusiasm stems from the belief that now that the Russians know the Americans are serious about maintaining an equilibrium of forces, they will come to the negotiating table. He assumes the Russians and Americans are rational enough at this point to stop the arms race if equilibrium is established again.

For Mitterrand, and he believes for all of Europe, the balance of forces is a balance of time. Deployment of Pershing II and cruise missiles into West Germany will reduce the time that the United States needs to strike Russian targets with nuclear warheads from 20 minutes to 5 minutes by 1985. This, he believes, will lead the Russians to negotiate.

Only Americans can maintain the convenient fiction that there is a difference between "strategic" and "tactical" nuclear weapons, Mitterrand says. "Strategic" for Americans means weapons that will cross the Atlantic and blast their cities. For Europe's short distances, everything is strategic. That is particularly true for West Germany, Mitterrand notes sadly, a major country that has the greatest concentration of nuclear warheads inside and targeted on its territory and which has no independent control over the decisions about those warheads. This underlies the enormous psychological and political problems the Bonn government faces in carrying out the NATO deployment decision.

It is not only on nuclear strategy that Mitterrand is content to support publicly an American policy in a way that contrasts vividly with France's previous determination to be seen to be going it alone in world affairs. His decision to provide French troops to the U.S.-sponsored Sinai peacekeeping force to carry out the Camp David accords caught even French diplomats off guard, particularly since the Middle East was the area of the world where France had most insistently sought to carve out a special role.

Much to the chagrin of France's right-wing politicians, Mitterrand does not seem to be runnning into any official American criticism for his plans to nationalize five major industrial groups and 36 banks, moves which will leave about 17 percent of national production and nearly 100 percent of French-owned credit facilities under state control. Mitterrand's aides assert that the sectors being nationalized, which include arms manufacturing, electronics and heavy industry, were carefully chosen "because they are of national size and importance, and need protection against multinationalization." Mitterrand himself rejects an international "division of labor" decided upon in New York or Bonn.

The victory last week of his good friend Andreas Papandreou, Greece's new Socialist prime minister, suggests that Europe's Socialist parties represent a new force that can mobilize public opinion more effectively than the Social Democratic parties, which have slipped in Scandinavia and West Germany in recent years. But Mitterrand sees this as a mechanical result of the fact that the Social Democrats, whom he praises as true socialists, have been in or close to power long enough to have to accept the blame for the economic reverses of recent years. He studiously avoided predicting that Washington and Athens could now find a formula to avoid open conflict, despite their ideological differences, as Washington and Paris have been able to do under Mitterrand.

He believes that the Reagan administration is simply mistaken in its analysis of what is happening in El Salvador. The guerrillas there, Mitterrand says, are not externally controlled communists, but have turned to insurgency and communist support only because the United States keeps beating up on them. They are fighting for liberty from archaic and repressive social structures. But, Mitterrand says as the last strawberry disappears, he recognizes that a serious problem exists for the United States in Central America and that often before a leader has the chance to philosophize, reality will grab him by the throat.