The United States and the Soviet Union have piled up enough H-bombs to destroy each other - regardless of who strikes first.

But war planners on both sides still fear that a large part of their nuclear offense might be crippled in a surpirse attack, called a first strike.

To keep a first strike from looking tempting, both the United States and the Soviet Union have fortified and dispersed their offensive weapons so all of them could not be destroyed in a first strike.

Both sides have buried their land missiles deep in the ground and piled concrete on top of them. They have put other H-bombs inside bombers that would take off at the first warning of attack so they would not be caugh on the ground. And still other missiles have been sent to sea in submarines.

Rather than keep building and diversifying more and more of the offense this way so there is enough left over even after riding out a first strike, there by persuading the other country that it would not be worth starting a nuclear war, Carter is proposing that both sides get rid of some of the weapons that would be best for striking first.

For the United States, this is the MX blockbuster missile under development. For the Soviet Union, this is the SS-18 intercontinental ballistic missile which is now being installed.

Secretary of State Cyrus R. Vance took to Moscow last week a proposal for a series of trades: the MX for some SS-s, and a "we won't if you won't" offer to stop improving the accuracy of multiple, independently targetable re-entry vehicles (MIRVs).

The Carter package delivered by Vance represented a feeling of self-assurance by the United States that its accuracy was far enough ahead to allow the Soviet Union to keep its edge in sheer rocket brawn, called throw weight.

The Soviets rejected the deal with essentially this response: You want us to cut back on our best weapons while you keep all of your advanced modern ones in place, except for the MX which is not yet built.

Carter administration officiasl countered they had made a fair offer that would end up in "rough equivalence" of nuclear power while taking a lot of nervousness out of he current gun-at-the-temple situation.

The Soviet Union, under the Carter plan, would settle for 550 MIR Ved ICBMs but could have only 150 of its new SS-18 missiles in that force. The SS-18 has the throw-weight to hurl eight big MIRV warheads at the United States compared to the three smaller MIRVs in the nose of the American Minuteman III. The Soviets had planned to deploy 308 SS-18s under their modernization program, according to U.S. officials.

Limiting the number of heavylift SS-18 type ICBMs in the Soviet land-based MIRV force would, according to U.S. specialists, reduce the Soviet throw-weight advantage from 3 to 1 to 2 to 1. Carter is counting on superior U.S. MIRV technology to bring up the balance to "rough equivalence."

To help ensure that American accuracy continues to offset the Soviet edge in throw-weight, the Carter proposal would make it harder for the Soviets to close their MIRV gap.

Perfecting a highly accurate MIRV requires a lot of experimental flight tests. U.S. weapons specialists persuaded Carter that limiting Soviet flight tests to six a year would keep the American MIRV lead until around 1983. Both America and Russia would limit themselves to six tests a year under the arms control proposal.

Soviet military leaders are expected to protest hotly such restraints on catching up on MIRV. If their opposition becomes crucial to an agreement, Carter could go back to a suggestion made while the arms control proposal was being formulated - letting the Soviets conduct 12 flight tests to the U.S. six.

Besides having to catch up with the United States in MIRV accuracy, the Soviet Union still has a ways to go to put MIRV heads on 550 ICBMs to match the Minuteman III force already on the line. The Soviets are believed to have MIRVed about 300 of their missiles.

If the Soviets would agree to the American proposal, U.S. defense officials argue, both Washington and Moscow could breathe easier because of the reduced threat of a surprise nuclear attack. But if both sides persist in their development of more accurate and bigger ICMs like the MX and SS-18, the hair-trigger nuclear strategy of "launch on warning" is expected to gain adherents in both countries.

Carter's other big offer - a swap of short-ranged American cruise missiles for short-ranged Soviet bombers designated Backfire - are beyong the central focus of reducing the first-strike threat. However, the trade is considered crucial to getting military leaders in both countries to accept the whole package.

U.S. Navy leaders insisted that they needed a short-ranged missile to use against enemy ships in wartime. The Air Force wanted a cruise missile with enough range to allow its bombers to stay outside enemy defenses. The cruise missile, which is like a drone, or pilatless, airplane, would carry the bomb from the bomber to the target.

In exchange for allowing short-range cruise missiles to be deployed, Carter proposed that the Soviets be allowed to deploy their so-called Backfire bomber as long as its range was also limited. The U.S. Joint Chiefs of Staff have argued that the Backfire should be counted as a strategic weapon since it has enough range to bomb the United States and then land in a nearby country like Cuba.

The chiefs credit the Backfire bomber with a combat radius of 3,100 miles. Therefore, North Atlantic Treaty Organization nations would be in easy striking distance of the Backfire. But U.S. cruise missiles could be launched from NATO nations against Wasrsaw Pact countries.

Although Carter has not disclosed how much range the cruise missile would have under his proposal, sources said that one number put on the table in Moscow was 2,500 miles. This kind of range would give Soviet leaders concern because such a missile could penetrate the Soviet Union just as the Backfire could penetrate NATO.

Although the cruise missile could fly farther than 2,500 miles and carry a nuclear warhead, arms specialists do not consider it any match for present day ballistics missiles in the strategic role. The cruise missile is slow, thus vulnerable to anti-aircraft defenses and cannot carry as big a warhead as ballistic missiles.