THE UNITED STATES, as it enters the 1980s, is rebuilding the military arsenal of the Vietnam War era. Most of the planes, ships and armored vehicles used in that war have been lost, used up or are out of date.

In the 1960s, the United States had a national strategy that called for fighting two big wars and one little war simultaneously. The Vietnam war, if it showed nothing else, proved that this 2 1/2-war strategy was unrealistic. Former Defense Secretary Melvin R. Laird, when he took office in 1969, dropped down to a 1 1/2-war strategy -- the capability to fight Russia and a minor power at the same time. That 1 1/2-war strategy is still in force today.

However, in terms of the number of troops who could be sent to war right now and many of the weapons they would have to fight with, the United States is worse off today that it was when its forces left Vietman in March, 1973. Here are some comparisons between 1973 and today.

Troops -- 2.25 million people on active duty compared to 2 million today.

Warships -- 438, incldding 16 aircraft carriers, compared to 373 now, including 13 aircraft carriers.

Bombers -- 500 to 415.

At first glance, one might conclude that it is just a matter of time and money before the United States rebuilds its arsenal. But this is not so. Neither the 3 percent nor 5 percent increse in defense spending that Congress has been arguing about will be enough to build the arsenal military leaders say they need to carry out the 1 1/2-war strategy.

The reason is that the weapons being built today to replace those used in Vietnam are so much more sophisticated that they cost several times as much. Even under generous Pentagon budgets, the Army, Navy, Air Force and Marine Corps will not be able to buy the quantities desired. Aggravating the problem is the tendency to offset rising costs by reducing the limited number of weapons on order.

To cite one of many examples, the F4 fighter bomber that was the workhorse of the Vietnam war cost about $6 million in today's dollars. The F15 fighter that is replacing it costs about $18 million. It is just plain arithmetic that the Air Force could buy fewer F15s than F4s with the same billion dollars. And this is the heart of the numbers problems.

The Air Force's F15 fighter, the Navy's Nimitz unclear-powered aircraft carrier, the Army's XM-1 main battle tank and the Marines' LHA amphibious warfare ship are all amont today's superweapons -- designed to be the best money can buy. Yet military leaders are supposed to worry about the worst possible threat -- meaning going to war against the Russians, not the Iranians or the Cubans. So they order weapons which could destroy the best of the Soviets' arsenal. In short, military leaders worry more about the one than the one-half in arming their services to carry out the nation's 1 1/2-war policy.

Buying the best has problems beyond the initial cost of the weapons, with the F15 fighter plane a current example. The F100 engines inside the F15 fighter push the airplane through the air at unprecedented sepeds, whether close to the ground, at high altitudes or in dogfight maneuvers. But this performance is so demanding that the engine has experienced stalls in mid-air, and its parts wear out faster than anticipated. This often grounds F15s sent to Europe to battle the soviets if war breaks out posing a risk to the North Atlantic Treaty Organization. Air Force leaders now admit they tried to get too much out the F15's engine, that they should have settled for a simpler one for more reliability, and less cost. t

Rep. Les Aspin (D-Wis,), a former Pentagon analyst and now one of its severest critics, recently addressed the military's penchant for ordering the best weapons money can buy Zeroing in on the Navy's carrier based warplanes, he found that, expressed in today's dollar, the Navy bought 88 planes averaging $6.1 million each in 1970. In 1980, he found the Navy bought only 51 planes at an average cost of $26.8 million each.

In ohter words, compared to the Vietnam war era, the Navy is spending four times as much today to buy one-third fewer fighter planes. "At this rate," Aspin complained, "We are paying more money for less security. The effect is the same in airplanes, ships tanks, missiles, all types of weapons. We are paying more and getting less. True, we are getting more sophisticated weapons for our money, but would anyone seriously argue that their improved performance is even nearly proportional to their increased costs? Clearly, we have to change our weapons procurement philosophy. We have to start buying airplanes that are simpler, cheaper and more reliable -- and buy more of them."

Granted, Aspin is a Pentagon gadfly. However, his views are shared by the sharp-pencil executives at the General Accounting Office. The GAO, in a recent report, fingered the culprit for rising costs as over-sophistication of weapons. "The desire of the U.S. military leaders to push the state-of-the-art with new concepts and designs has the biggest effect on cost," said Congress' watchdog agency.

To buy all those fancy weapons the military services have put on their shopping lists, GAO estimates, would require Congress to double the Pentagon's procurement budget every year for the next 10 years. A 5 percent annual increase would not come close to providing the $725 billion GAO estimates would be required over the next 10 years to buy the weapons on the wish list.

Since no one is talking about doubling the Pentagon's hardware budget in peacetime, the next result will be not enough weapons to carry out the existing war plans. The United States has to take risks, change its war plans by scaling back its international commitments, or find ways to buy more weapons with the limited amount of money available.

Although one can scold the generals and admirals for taking the nation to the poorhouse in Cadillacs as they choose their weapons for the 1980s, civilians are responsible.

"The Congress shall have power to provide for the common defense," states the Constitution.

There are few signs that the majority of the Congress is ready to revolutionize military procurement to obtain more weapons for the same amount of money. For example, despite President Carter's veto of last year's defense money bill to stop Congress from providing funds for another $2-billion Nimitz nuclear-powered aircraft carrier, a majority reversed him this year and insisted on this Cadillac of the sea. Carter had recommended building small carrier -- a Chevrolet -- to save money.

Although the congressional military committees often add to the cost problem by insisting on continuing to produce weapons the active duty military does not want -- weapons like the Vought A7 attack plane championed by the Texas delegation -- lawmakers outside that loop complain they have trouble making responsible judgments on what weapon should be purchased.

Said Chairman Jack Brooks (D.-Texas) of the House Government Operations Committee as he opened this year's hearings of the rising costs of weapon: "It is a fact that since 1969 the Department of Defense has underestimated the costs of all major systems by more than 50 percent. When Congress has been asked to fund weapons systems over the last 10 years, we have made decisions based on consistent underestimates of the actual costs. This fact, in many ways, effectively deprives the Congress of the ability to make sound decisions on behalf of the citizens that ultimately pay these greatly incrased costs."

Despite such confusion, the specter of a group of Iranian students holding the mightiest military power on earth at bay for weeks may stimulate some rethinking. The existing Nimitz aircraft carrier has been ordered to leave its station in the Mediterranean next week to relieve the Kitty Hawk steaming off Iran. The Navy does not have enough ships to cover both the Mediterranean and Indian Ocean properly.

Former Navy Secretary W. Graham Claytor Jr., in vainly urging Congress to approve a cheaper carrier so three of them could be purchased for the price of two Nimitz giants, stressed that no one ship can be in two places at once. The same thing is true of warplanes and tanks.

Also, they would be lost in large numbers in any major war. Throwing more money at the Pentagon, without changing the way it is spent, would seem to be wrong way to go in the 1980s.