Defense Secretary Caspas W. Weinberger said in his first message to the troops yesterday that his mission is "to rearm America."

"As I take office as our 15th secretary of defense," Weinberger said in a message to the 3 million military and civilian people attached to the Defense Department, "I am very much aware of the need to add greatly to America's military strength."

The first solid indication of how he intends to do this will come in the way he handles the fiscal 1981 and 1982 defense budgets inherited from the Carter administration.

Weinberger's options are to let the Carter budget stand pretty much as is, change it only slightly or attempt a complete overhaul, Edwin Meese III President, Reagan's counselor, said last week that the new administration might be able to live with the fiscal 1981 defense budget it inherited.

Maj. Gen. Jerry Curry, Pentagon spokesman, said yesterday the Weinberger team is reviewing the defense budgets and hopes to have any revisions ready to mid- or late February.

At his Senate confirmation hearing, Weinberger said he did not believe in fixed percentage increases in defense budgets. Congressional conservatives insisted that the Carter administration raise the defense budget by 5 percent a year, after allowing for inflation. Weinberger seems to be distancing the new administration from any such set standard.

Carter's fiscal 1982 defense budget calls for big increases to make military forces readier to fight, an emphasis Weinberger endorsed at his confirmation hearing and in his first message yesterday. He pledged as well yesterday "to begin now to restore our strategic balance."

Reagan's condemnation of the pending strategic arms limitation treaty (SALT II), which took seven years to negotiate, and Weinberger's pledge to rearm the country contrast with a major theme in former defense secretary Harold Brown's farewell report released this week.

"As for the overall strategic balance," Brown said, "it is my judgment that the United States and Soviet Union remain essentially equivalent." He added that it is "imperative" to modernize forces as Carter has proposed "to preserve this rought balance for the remainder of the decade."

Brown said he remainded convinced that SALT II, as signed by Carter, is sound. He said the limits it would impose on Soviet strategic weapaonry would "make it easier and less expensive for us to maintain essential equivalence in the future."

For more than two decades, Brown has contended that it is highly unlikely there could be such a thing as a "little" nuclear war. "It remains my belief," he said in his final defense report to "Congress, "that a limited exchange is unlikely to remain limited."

Turning to some chilling statistics, Brown warned that a so-called "limited" Soviet nuclear attack "on our ICBM silos alone" could kill 2 million to 22 million Americans within 30 days as the fireball, blast and radioactivity fallout took their toll beyond the 1,054 targeted Minuteman and Titan missiles standing underground.

If the Soviets launched all-out nuclear attacks on each other, not sparing cities as in a so-called "limited" nuclear exchange, at least 20 million and as many as 165 million people would be killed, Brown said. The horror would be similar in the Soviet Union, with the death toll ranging from 23 million to 100 millon people, Brown said.

As if that were not enough, Brown added in the flat prose of the Pentagon:

"Beyond this, secondary and indirect disruptions of the societies attacked, and longer term fallout and other consequences to areas outside those attacked, would amplify the damage.

"Deterring nuclear war -- making that unlikely possibility even more remote -- is therefore our highest national security priority."

Brown's report contains the latest intelligence estimates on how many nuclear weapons the United States have aimed at each other. Adding up the two arsenals, there are 16,000 deadly H-bombs ready for launch. Here are the 1981 estimates of U.S. and Soviet nuclear forces: (TABLE) (COLUMN)*2*U.S. Soviet Land Missiles(COLUMN)1,054(COLUMN)1,398 Submarine missiles(COLUMN)576(COLUMN)950 Long-range bombers(COLUMN)347(COLUMN)156 Nuclear warheads(COLUMN)9,000(COLUMN)7,000(END TABLE)