BOTH PRESIDENT CARTER and President Reagan have threatened to go to war to save Persian Gulf oil fields. Could our military really do this? I say let's find out in the New Year.

The way to find out is not through the carefully planned and rehearsed military exercises of the past, like the recent Bright Star 2 mission to the Middle East and Africa which only involved about 6,000 troops spread thinly over four different countries. The Pentagon knew full well ahead of time that the exercise could not fail.

Let's do something realistic, an exercise that bears enough resemblance to the real thing so we all know how good or how bad we are at putting out fires that suddenly break out 10,000 miles away. "It would tell the policy makers what the generals don't know: whether we can answer the fire bell," said one Army colonel who endorsed the idea.

Here is my proposal, set down after consultation with some military specialists who also would like to know where we stand, before they are sent to some hostile desert thousands of miles from any friendly base:

Assume that the oil fields in Iran are in danger of being taken over by the Soviets and their surrogates. Intelligence has warned that a Soviet armored column is forming up near Iran's northern border at the same time the Iranian military is moving to occupy Iranian oil fields. Assume further that the American president wants to rush a strong enough force to the scene to freeze the situation and, if negotiations fail, take over the oil fields.

There are paper plans lodged with the Joint Chiefs of Staff to deal with just such a situation. But, unless these plans are tested realistically, thousands of Americans might die needlessly because nobody anticipated the real-life problems they and their weaponry would run into once they went into extended combat on a distant desert. If the Iranian rescue mission proved anything, it proved that what can go wrong usually does go wrong on military operations.

Since foreign governments in the Persian Gulf area would not approve an American military exercise on their ground that would be big enough to be realistic, I say conduct the test right here at home. The United States has deserts just as challenging to troops and weaponry as ones in Iran or Saudi Arabia. And getting Marines and paratroopers from one coast of this country to another would be a fair approximation of the problems of doing this in the Indian Ocean area.

Specifically, establish a bright military planning group to work out the details around this general scenario: The contested oil fields are at specified spots on three military bases in Southern California -- the Marines' Twenty Nine Palms training ground in the Mojave Desert, the China Lake Naval Weapons Center southwest of Death Valley, and the inactive Fort Irwin Military Reservation. Different U.S. outfits will be designated as aggressors and defenders in as realistic a struggle as can be devised for these oil fields. Monitors on the scene will determine who won and who lost, and why.

Because the Marines are expected to be the spearhead of any U.S. action to save Persian Gulf oil fields, they should play a big part in the exercise. My suggestion is to designate Marines already stationed at Twenty Nine Palms as the native military outfit which has taken the oil fields away from its government and asked the Soviets to block any American attempt to intervene.

For realism, again, I would designate a U.S. armored force to act as the "Soviet" one, complete with as much heavy equipment as environmentalists would allow on the deserts of California. The "Soviet" armored force would be an American armored brigade in Europe suddenly ordered to fly to a staging area in Nevada, perhaps an airstrip on the vast Nellis Air Force Range, tanks and all. From Nellis, the "Soviet" column would drive across the mountains and desert to the oil fields at China Lake and Twenty Nine Palms, providing a valuable test of men and weapons along the way. If environmentalists objected to tank columns crossing Death Valley, jeeps could be used as "tanks" for this part of the exercise.

If 10 percent of the horror stories I have heard over the years about our readiness are correct, the designated European brigade would have to strip the rest of its division to show up at Nellis with the equipment, ammunition and spare parts the book calls for. Let's find this out, too, while there is time. And flying the brigade east to west would also test the state of our military airlift.

My candidates for the American rescue forces are some of the ones already assigned this fire extinguisher role as part of the Rapid Deployment Force: amphibious Marine units from Camp Lejeune, N.C., and the Army's 82nd Airborne Division from Fort Bragg, N.C. I would let the Marines show what they could do coming toward the desert "oil fields" from the sea, landing, say, near Oceanside, Calif., just outside the Marine base at Camp Pendleton. No fair stocking up on food and water and ammunition at Pendleton. The Marines have advertised themselves as ready for a come-as-you-are war. Let's see how ready.

After coming in over the beach near Oceanside, no mean feat in itself given the hilly terrain around there, the Marines would drive overland toward the oil fields with as much heavy stuff as California officials would allow. They might have to travel at night until they reached the desert to keep from clogging up main roads. But there must be a reasonable and realistic way to run this part of the test. We're not talking, after all, about laying an Alaskan pipeline -- just a test of what we could really do in those distant trouble spots.

The 82nd Airborne would be given only as much warning as might be expected from a sudden crisis in the Persian Gulf. How soon could the full division, not just a few parade units, get from one coast to another with enough water, beans and bullets to fight 30 days without resupply? One former 82nd Airborne officer predicted it would take far longer than advertised -- again, something worth knowing for generals and policy makers alike. The 82nd could be landed east of the oil fields in Arizona and drive overland from there in another challenge to desert troops, tactics and equipment.

Because so many Persian Gulf hotspots are far from U.S. air bases and out of range of carrier-based aircraft, I am tempted to leave aviation out of this exercise. But it would not hurt the lessons of the exercise if warplanes were included under a realistic scenario. And I would not give any of the combatants the option to use nuclear weapons. However, a lesson might be that survival of one side or the other boiled down to using nukes.

To test today's ability to keep the fighting man on the hot desert supplied with water, ammunition and food, I would make sure the exercise was no overnight affair. The troops would be kept engaged for about six weeks so they could survive on just what they had brought along. Let's see how well our logistical tail, which gets a big slice of the military budget, can swing into action. The president and the generals need to know.

The post-audit of the exercise would be conducted by merciless "murder boards" of specialists outside the chain of command of the participants. Their task would be to determine what worked, what didn't, and why. Everyone from sergeants to generals would have a chance to describe into tape recorders what they found out about their men and equipment out on the desert. Their uncensored remarks would be transcribed for study by civilian policy makers, military commands, Congress and, with minimal deletions for legitimate security reasons, the public. Lessons from such past computer readiness exercises as Nifty Nugget have been kept secret. Besides, how well or poorly armies do on paper does noSot tell you much about how they would do on the battlefield. Let's look harder this time before we leap, by using a large- scale desert exercise.

Sure, it would cost money. That manicure of an exercise known as Bright Star 2 cost about $69 million, without telling policy makers much of what they need to know. If money for the exercise looms as the big problem, why not cancel that other rote annual drill called Reforger, which sends American troops to Europe to do the same things year after year? Or are we afraid to find out what we can actually do about saving Persian Gulf oil? It's time to focus a bright light on our capabilities in a laboratory where the hard facts cannot be hidden.