President Reagan yesterday guaranteed a continued national debate about defense spending by asking Congress to appropriate a sizable down payment on a new five-year military buildup. It would require $1.77 trillion in budget authority and $1.55 trillion in actual spending from fiscal 1984 through 1988.

Defense Secretary Caspar W. Weinberger and Gen. John W. Vessey Jr., chairman of the Joint Chiefs of Staff, said in separate reports that the United States must spend such record-high, peacetime amounts in response to a relentless Soviet military buildup.

Soviet forces have grown so strong, Weinberger said in his annual report to Congress, that "they now seem capable of mounting roughly concurrent offensives against western Europe and southwest Asia.

"If Soviet conventional forces attacked or tied down allied forces in these two critical theaters," he said, "North Korea might then take advantage of the situation and launch an offensive with their numerically superior ground and tactical air forces against the Republic of Korea."

While stressing that the Reagan rearmament program calls for bolstering non-nuclear weaponry of the Army, Navy, Air Force and Marine Corps--partly to forestall any inclination to engage in nuclear war--Weinberger also outlined an $18.2 billion program to beef up the strategic forces for waging nuclear war if deterrence fails.

About one-third of that total would go for the MX intercontinental ballistic missile, for which a basing plan is to be recommended this month.

Weinberger's rhetoric about waging nuclear war was softened markedly in comparison with his public report last year and his secret guidance to the military services. The guidance included statements about preparing for a "protracted" nuclear war, a concept that critics said was dangerous and unsound.

In what defense officials said was an attempt to reassure members of the anti-nuclear movement in the United States and abroad that the administration is not about to be reckless, Weinberger's report said:

"We for our part are under no illusions about the dangers of a nuclear war between the major powers; we believe that neither side could win such a war . . . . There is nothing new about our policy."

Vessey, in filing his first report on behalf of the Joint Chiefs, said the death of Soviet leader Leonid I. Brezhnev last November "raised hopes that a less aggressive foreign policy might be forthcoming . . . . There is as yet no indication that the new Soviet leadership" under Yuri V. Andropov "plans any such major shifts in policy."

Reagan's newest five-year program calling for $1.55 trillion in spending goes to an increasingly resistant Congress, which has shaved his original request from $1.289 trillion to $1.223 trillion for fiscal 1982 through 1986.

President Carter had recommended spending $1.173 trillion in that period. Shortly after coming to office, Reagan added $116 billion to that amount, which now is down to $50 billion, much to the distress of Reagan and Weinberger.

In his new five-year military spending plan, Reagan is asking these amounts: for fiscal 1984, $238.6 billion; 1985, $277.5 billion; 1986, $314.9 billion; 1987, $345.6 billion, and 1988, $377 billion.

The yearly increases in those five fiscal years would average 12.4 percent without allowing for inflation. Adjusted for inflation, the annual increases for fiscal 1984 through 1988 would average 7.2 percent, or double the 3 percent real growth in military spending that the United States and its North Atlantic Treaty Organization partners pledged during the Carter administration.

The question that Reagan's new budget puts before Congress is whether the nation can afford such increases in defense spending at a time when federal deficits are projected to be more than $200 billion a year. Congress has been reluctant to cancel weapons that the president has portrayed as vital to the national defense, partly because this would mean eliminating jobs.

The lawmakers have been more inclined to reduce funds for operation and maintenance of existing planes and ships, the so-called readiness accounts, or to impose general percentage cuts in the defense budget and leave it to the Pentagon to decide where to impose them.

Sen. Carl Levin (D-Mich.), a member of the Senate Armed Services Committee, is among those who wants to cancel such super-weapons as the MX missile and B1 bomber on grounds the money would be better spent on conventional forces.

Despite calls from Republican and Democratic leaders for military spending cuts of $15 billion to $20 billion, Levin reflected what seems to be the majority view of those involved with the defense budget when he said in an interview:

"Congress will be a little more independent this year, but I still see tinkering, not wholesale cuts. Congress is afraid of being painted anti-defense."