The technological revolutions in modern warfare and modern communications came together this week in a succession of thunderclaps as shattering, in their way, as television's first, forced entry into our living rooms with the Vietnam War.

I'm speaking only of the likely initial impact of CBS's admirably ambitious five-part documentary, "Defense of the United States" -- of the skill and force with which it sought to focus public perceptions of the great war/peace issues of our national security. Its enduring influence, of course, may be something else. In the case of Vietnam, after all, a war was on.

"Defense," on the other hand, was addressing itself, hypothetically, to future wars, for which there is, inevitably, a shorter attention span. It dealt, stupefyingly, with plans and programs; with strategies and contingencies; with only educated guesses of enemy intentions and rough estimates in round numbers (mostly millions) of casualties.

But along the way it said plenty about real things. The thought processes of American military men and of the engineers and scientists involved (the systems analysts) were laid bare, in their own words. The awesome American arsenal in being or on drawing boards was vividly displayed. Taxpayers were invited to contemplate the cost -- not just now, but the projected, skyrocketing, mind-bending future cost -- and the potential economic toll of this country's supposed defense needs.

The effect was remarkably balanced and free of judgments, except for one: by the magnitude of the effort and its timing, what CBS was saying was that it thinks the American public may be readier than it has ever been in peacetime to question profoundly the underlying assumptions and long-range implications of current national security policy.

The CBS commitment to a five-part (one hour ech) prime-time documentary on "Defense" is not the only evidence. It is merely the most impressive evidence so far that the Reagan administration's nearly unimaginable, five-year, $1.5 trillion rearmament program may be moving us to the front edge of a debate over national security fully as intense and contentious as the controversy over the administration's program for the economy.

It may be slower developing. For this year, the Reagan defense budget is long on obligations for huge increases in spending in the future, short on immediate shifts in emphasis or sudden boosts in outlays. It is even shorter in having a sense of grand strategy; unlike domestic policy, up to now there has not been much of substance to chew over.

True, critical and immediate decisions confront Secretary of Defense Caspar Weinberger on the MX missile and the new strategic bomber, and there are more tough procurement choices down the line. But as Newsweek noted in an exhaustive cover story on "Reagan's Arms Buildup" the other day, "for all its revolutionary impct, the massive defense buildup planned by the Reagan administration . . . has so far drawn little dissent."

Technically, that's so. But the Newsweek treatment was in itself another pice of evidence that a debate is getting under way. In it, sharp questions were raised about how manpower needs can be achieved without a return to the draft; about the inflationary impact; about "the lack of a coherent strategy."

The administration, said Newsweek, "is seriously risking a mismatch between its stated goals, the kinds of weapons it is building and its developing strategic concepts. In the long run that could prove enormously damaging -- both to an inflation-prone economy and to the powerful but fragile political consensus that supports a vast new defense effort and remains essential to its success."

Other criticism, equally sharp, has somehow been all but lost in the shuffle of domestic issues and a generalized public support for more defense, and never mind what, or precisely why. Economist John Kenneth Galbraith, for example, has delivered more than once a set speech deploring the arms race and urging greater efforts to come to terms with the Soviets on arms control.

"Those who say that we should accept the risk of nuclear conflict to save our system are saying, in the strongest possible terms, that we should accept its certain destruction," Galbraith argues.

Elder statesman and Soviet expert George Kennan made a little splash with a bold proposal for a 50 percent cutback in American and Soviet nuclear arms. Another former ambassador to the Soviet Union, IBM executive Thomas J. Watson Jr., made no spash at all with a reasoned and persuasive Harvard commencement address decrying what he called "thermo-nuclear McCarthyism" -- the charge that "anyone who favors an end to the arms race must be soft on U.S. defense or even soft on communism."

But that was before CBS's "Defense." Its five-hour exposition of the issues will settle few arguments. But it may help start some, which would be contribution enough. Dan Rather observed at the end of the fifth segment, "We're heading toward the largest military buildup in this nation's history, with few questions asked."