Why France no longer needles Uncle Sam

France, a neutral country in all but name, has discovered a monster that must be destroyed by all means if Europe is to be saved - neutralism. That is the message French President Francois Mitterrand conveyed during his recent visit to West Germany when he warned against decoupling Europe from the United States.

Mr. Mitterrand could speak with authority because between France's implicit neutralism, based on the threat of nuclear retaliation should French territory be attacked, and the neutralism threatening West Germany, there is a substantial difference. The French brand is based on a will to fight in its defense; the German variety is a renunciation of the use of force under all circumstances.

Were European neutralism to assume such proportions that the US considered the continent no longer worth defending, France would feel directly threatened because its nuclear capability would cease to be credible. The present deployment of Soviet SS-20 missiles, if left unchallenged, gives the Soviets an offensive capacity that France's defensive capacity cannot match.

French strategists have warned repeatedly that the accuracy of the SS-20s is such that they could knock out the 600 or so Western strategic and defense installations with one blow. Because they are so precise the missiles' destructive yield can be reduced to the level necessary to achieve their purpose , thus causing minimal collateral destruction and loss of human lives.

''Should a few tens of thousands of civilians die following a Soviet attack, '' a French strategist stated, ''is it conceivable that a French president would order a nuclear retaliatory attack on the Soviets causing the death of several million people?''

''That is not credible, especially considering that a Soviet counterretaliation would knock France off the map,'' he added. These considerations take into account a fundamental difference between the Soviet and the French nuclear systems.

The Soviets have an anti-force capacity which aims at destroying an enemy's nuclear installations. The French lack the means of acquiring such a capacity; their nuclear strategy is anti-cities; that is, based on the threat of massive retaliation.

This fundamental strategic difference explains why President Mitterrand and British Prime Minister Margaret Thatcher have categorically rejected Soviet offers to reduce their number of SS-20s to match the combined French and British missiles deployment.

Moscow's proposal would leave the Soviets with offensive and the British and the French with defensive weapons - hence there could be no question of parity.

When asked why, if the Soviets are capable of disarming Europe with one blow, they don't go ahead and do it, French strategists say the basic reason is fear of the American reaction. The Soviets cannot be sure that the US would not go to war and start a nuclear exchange.''Even if the chances were only 5 percent,'' one French strategist said, ''that would be five points too many considering the awesome risks.''

Because Moscow has reasons to fear the US and the US only, France would consider the lack of an American commitment to the defense of Europe an incalculable and irreparable disaster.

Mr. Mitterrand has warned the Germans that neutralism risks the decoupling of Europe and the US without war. The Soviets hope to scare the Europeans into submission and are therefore determined to preserve the wherewithal to scare them. That is why they will accept a reduction of armaments so long as the balance remains in their favor.

When the Germans were the West's bastion in Europe, the French amused themselves with pricking Uncle Sam and missed no occasion to proclaim their much vaunted independence.

Now that that bastion shows cracks, the French have become the champions of the American presence in Europe and of preserving the nuclear balance in order to keep the peace.