How could the 13-nation North Atlantic Treaty Organization (NATO) quickly increase its forces in Europe by 500,000 troops, more than 1,000 tanks and some 500 jet warplanes?

The answer is somehow to convince France to rejoin the military part of the NATO alliance that President De Gaulle took her out of in 1966.

To be sure, even if French President Giscard d'Estaing wanted to bring France back into the NATO military alliance, and there is no evidence that he does, the political odds in France against such a move would be very large.

Nevertheless, much has changed in the past 13 years and, while the effort to bring France back into the alliance may prove futile, it is too important a question to allow to sit forever idle.

For example, France's reenlistment could have the following effect:

It could ease the pressure on West Germany that is being steadily exerted these days by the Soviet Union. Without France in the military alliance, the 500,000-man West German armed force remains by far the only big, well-equipped West European fighting force that is actually on the continent and committed to European rather than just German defense.

As such, the Soviets continually single out the Germans as their main target for troop reductions at the joint Western-Soviet bloc troop reduction talks in Vienna. In addition, the Kremlin, for the past two years, has engaged in a skillful policy of courting Bonn with improved relations on one hand, while warning the Germans not to allow new U.S.-supplied neutron weapons or other modernized atomic weapons on their soil.

West Germany, which wants to maintain good relations with the Soviets, is therefore under tremendous pressure and has no one who can help deflect that pressure. The British have only about 55,000 troops here. The Belgians, Dutch and the rest of the small NATO European countries have a numerically insignificant presence in forward positions.

France is the only other continental power with an army of equal size that could help the Germans in terms of diffusing the Soviet diplomatic offensive.

Since the end of the draft in the United States in 1974, the U.S. Army's reserves have virtually disappeared to the point where the Army is 100,000 or more men short of the reservists who would be needed in Europe in the early stage of a war. French troops, committed once again to NATO, could help plug that gap in men needed to defend what is, after all, European soil.

A renewed French military commitment would also formally change the balance of force statistics that Soviet marshals would have to consider when plotting the odds of success of conventional war in Central Europe.

It can be argued that Soviet planners already take the French forces into account in assessing possible future battle plans and that they believe the French would fight.

Yet what about a Soviet attack that was intent on overrunning Germany and with announced plans not to cross the French border. Would the French join in earlier?

Nobody knows what the Soviets would guess. But the French forces are formidable and their rejoining the military alliance would clearly increase the known number of forces the Soviets would have to contend with and perhaps add significantly to overall West European deterrence against attack.

In a recent book called "The Third World War, August 1985" by retired British Gen. Sir John Hackett, the French armed forces play a crucial role in helping the West eventually turn back a Soviet attack that has evolved out of a severe future crisis.

The book, of course, is fiction and it is very hard reading unless you enjoy Pentagonese. Nevertheless, it raises some important points.

Specifically, in the fictional account, the French forces, which eventually join the battle, play a key role as reserve troops in an allied counterattack in Western Europe. In Gen. Hackett's account, the most crucial blunder the Soviets make is guessing that the French troops won't be committed to battle and, he wonders, whether the Soviets would have launched their attack at all had France rejoined the military alliance before the war and the Soviets known for sure, therefore, that the French would fight.

Finally, there is the likelihood that the return of France would give the alliance itself an important and badly needed shot in the arm, show a greater European willingness to come to its own defense as a West European entity, and provide something to help balance the decline in confidence in the United States that is -- fairly or not -- widely felt in Europe.

There are French officials who will counter all of these points.

The decline in confidence in U.S. leadership in recent years, they will say, in fact shows that DeGaulle was right when he pulled out of the military alliance for similar lack of confidence -- at that time over the U.S. nuclear pledge to protect Europe.

French conventional forces, they will say, will not be allowed to come under the American commanders who dominate NATO. And French nuclear forces must remain exclusively under French control. Unstated is that the French also probably enjoy their independence and status as, in effect, the world's third-ranking all-around military power.

Finally, they say, it is the nuclear commitment that NATO needs most to keep Russians away and not additional ground forces.

Yet, the fact is that none of these issues has been aired publicly in a long time and it may be that French opinion has changed to some degree or that France's continental allies -- especially West Germany -- can play more of a part today than the United States can in coaxing the French back in as a matter of mutual interest.

Indeed, though it is clear that the Carter administration's image in Europe is suffering just as it is in the United States, the president has kept his promises to NATO in terms of beefing up both U.S. manpower and firepower and has reaffirmed the U.S. nuclear pledge. Though nuclear deterrence is vital, it has long been acknowledged that NATO's most obvious weakness is in conventional forces, especially reserve forces behind the front lines, and that is precisely what the French have.