"THE DEFENSE of the United States" promises to be not only the most important but also the most fascinating TV documentary of the year. CBS is devoting five hours, spread over five consecutive nights starting tonight at 10, to the subject of defense and related topics like, oh, The End Of The World.

The United States will spend $1.3 trillion on defense over the next five years. CBS News spent just over $1 million on what may well be the first documentary epic in TV history, one more notable for its quality than the size of its budget. The program -- exhaustive, engrossing, alarming -- reaffirms the obvious, that CBS News remains the leader among the networks and that neither of the other two could or would have attempted a report on this scale and of this depth.

"Defense" proves itself worthy heir to "Harvest of Shame," "The Selling of the Pentagon," "See It Now" and the other broadcasts CBS News people adore pointing to with pride. "Defense" was produced under the "CBS Reports" banner which coincidentally began -- in the legendary hands of Edward R. Murrow and Fred W. Friendly -- with "Biography of a Missile" in 1959.

The program also proves that the TV documentary is not dead as an informational format, that complex subjects can be covered at length on television, and that not everything told viewers needs to be whittled down to bites the size of Wheat Chex.

Already wildly timely, this sprawling omni-doc has been given added oomph by (and will undergo some eleventh-hour editing to take note of) last week's stunning Israeli raid on an Iraqi nuclear reactor. We are living in a world of geometrically multiplying unthinkable scenarios; "Defense" makes many of them as clear as day and real as night. As television and as a national conversation piece, it is a substantial, powerful and potentially valuable achievement.

The first hour is the best and flashiest, partly because, thanks to $87,000 worth of special effects manufactured in Hollywood, the producers succeeded in wiping Omaha, Neb., off the face of the earth. Dan Rather, anchor for the entire series, visits the Strategic Air Command in Omaha for a 53-second demonstration of what would happen if one 15-megaton nuclear weapon exploded in front of SAC headquarters, a likely target. It would only take 53 seconds to level the city, Rather says.

"The fireball would remain on the ground for 20 seconds. And open a crater three-quarters of a mile across. It would then rise to a height of 80,000 feet in less than a minute, generating enough heat to cause second- and third-degree burns 16 miles away," Rather says. There would be 2 million dead within six weeks' time in and around Omaha. Two New York doctors, experts in the field, say that even in bomb shelters, people would be "dry-roasted as in a crematorium."

Earlier, Rather stands outside the North American Aerospace Defense Command, submerged within one of the towering Colorado Rockies, to point out that although the facility has been built to withstand effects of nuclear attack nearby, "a direct hit . . . would turn this 100-million-year-old mountain into an instant tomb." The words as well as the pictures in this report are shrewdly well-chosen and sharp-edged.

CBS News isn't out to scare the bejeebers out of everybody in America with this report, but a viewer is propelled forward through a dense thicket of information with all deliberate speed and extremely appropriate urgency.

"Ground Zero," the introductory first hour, is more than a parade of hardware and intimidating statistics. Realities of the nuclear age are given new intimacy. At a missile launch site in North Dakota, correspondent Bob Schieffer talks with a young crewman who someday may have to press buttons that would send missiles to target cities he does not and will not know the names or locations of. "I don't have a need to know, to start with," he tells Schieffer. "Secondly, I would feel kind of emotional about what kind of people I'd be destroying."

He is not overwhelmed by the magnitude of his position. "It's a definite challenge," he says, sounding a little like a bizarre recruitment commercial. "It's more responsibility than I could obtain in a civilian world. To me, that is job satisfaction."

Schieffer also gets to pose with, for the first time on television, an actual and exposed MIRV missile, each of its three warheads 25 times more powerful than the bomb dropped on Hiroshima. The program clarifies such terms as "limited nuclear war" and "first strike capability" and, more importantly, takes subjects formerly in the domain of sci-fi fantasy and makes them understandable, if not necessarily fathomable.

And what about what Schieffer calls "the ultimate nightmare," a nuclear mistake?Could the buttons be pushed and missiles launched by accident? Within one 18-month period, Schieffer says, there were 147 false nuclear alarms recorded, one of them triggered by a fire in a Siberian pipeline, another by a "failed computer chip." Could it happen? "Never," says a general, not very reassuringly at all.

The first hour was produced by Judy Crichton, senior producer Andrew Lack, and executive producer for the whole series, Howard Stringer. Stringer and Lack's most noticeable previous documentary was "Teddy," the double-whammy aimed at Sen. Edward M. Kennedy in 1979. Crichton also produced part two of "Defense," "The Nuclear Battlefield"; Maurice Murad produced part three, "A Call to Arms"; Craig Leake, a talented NBC News expatriate, produced part four, "The War Machine"; and Lack produced the final and, unfortunately, most disappointing segment, "The Russians," with Walter Cronkite making his first reportorial appearance on CBS since leaving the Evening News in March.

Also instrumental in bringing this monumentality to television were Cbs News President Bill Leonard, in whose brain the idea first hatched last August, and Vice President Roger Colloff, rumored a likely candidate to be Leonard's successor when he retires in a year or two.

Although the first night's broadcast is the showiest, there is much to see, hear and be appalled or enlightened by in succeeding programs:

Cameras filmed an armored division exercise by U.S. troops stationed in Germany last winter on what Brig. Gen. Niles J. Fulwyler calls a "modern-day integrated battlefield," but the maneuvers turned into a mess after a simulated atomic blast. "Your tank is dead," a captaim tells a tank commander who seems to have no idea what's going on. "I would appreciate it if you would get off the tank," the tank commander says.

Presciently enough, Harry Reasoner reports in part two that "it is the French who have been giving the superpowers fits when it comes to nuclear activity." The Iraqi reactor was largely French-built. Gen. Schlomo Gazit, former head of Israeli military intelligence, tells Reasoner that on the matter of becoming a nuclear warrior, "At least one country is today working very hard on it in the Arab Middle East. And that is Iraq." This segment was excerpted for use on the CBS Evening News last week after the Israeli raid.

In "Call to Arms," the shaky state of the American military is examined. The Army is short about 25,000 skilled noncommissioned officers, it is reported, and the Navy short 22,000 men with critical skills. "Why'd I come in?" asks a soldier getting his hair cut."Because there's no jobs. Can't find no jobs." Correspondent Ed Bradley's affably laid-back interviews with military men are top-notch.

In "The War Machine," the segment that will probably irk the military the most, boondoggles and runaway albatrosses like the F-18, "most expensive war plane ever built," and the Lockheed C5-A are scrutinized. As it is taking off on a test flight, a wheel falls off the C5-A. An Air Force commanders says, "With respect to the wheel coming off, "I don't like that. But I think that a glance at history will show you that wheels have come off nearly everything that wheels have been put on."

Walter Cronkite, interviewing Alexander Bovin, speech writer to President Leonid Brezhnev, asks him, "Is there a feeling here that the Soviet economy can afford as well as the U.S. economy this next round of the arms race which seems to be imminent?" Bovin replies, "Mr. Cronkite, where the question is of life and death, we can stand anything." Later Bovin, a dead ringer for William "Cannon" Conrad, tells Cronkite philosophically, "Like all fat people, I am an optimist."

In the Russian segment, Cronkite interviews dissident and historian Roy Medvedev; the KGB arrived three weeks later, Cronkite says, and interogated all Medvedev's neighbors. Soviet TV pictures of a Russian missile-designing bureau, scenes never seen here before, are shown, preceded by bizarre footage from a Soviet wargame -- two bundled-up comrades lugging a trunk full of secret code books to a hideaway from which World War III could be directed by Soviet officials.

But too much of "The Russians" is given over to a long schmooze session Uncle Walter, pipe in hand, held with Western reporters stationed in Moscow; the amount of information gleaned from this is insufficient to justify all the time spent on it.

Rather interviews Secrtary of Defense Caspar W. Weinberger for the report, but Weinberger, who showed up at the reception CBS News tossed for Rather here recently, is not very illuminating, and neither President Reagan nor Secretary of State Alexander Haig would agree to interviews.

Says Stringer from his New York office, "I think Haig heard somewhere that we were soft on the Russians," which the program, in fact, is not. "We had all kinds of problems with the Russians," says Stringer. "They fought us every step of the way." Cronkite even reportedly staged one of his famous temper tantrums in Moscow.

Former president Jimmy Carter agreed to be interviewed but then canceled the night before a CBS News crew was leaving for Plains. "The reason given was that his advisers decided this was an 'inappropriate forum,'" says Stringer skeptically. "I think it probably had more to do with memoirs and money.He didn't want to show his fast ball too soon."

Stringer says the nuclear age has taken its toll on the staff of 80 that worked on this production for the past eight months: "We have five wrecked marriages" attributable to long hours in editing rooms and in the field. But he also says the project has been like a Vitamin B-12 shot for his unit and for documentaries in general.

"The general euphoria around here right now is just quite staggering," says the British-born Stringer. "As we got into this, it made us realize, 'Hey, we are really in journalism.' We've been producing the occasional documentary around here for years, but this is the first time, probably since Murrow's old weekly show, that there's a sense of 'Now we've made it. We've proven ourselves. We count.'

"We're no longer just 'basket weavers,' as we were once called by the hard news boys."

Stringer does not argue with the assessment that neither ABC nor NBC would have tackled this topic in such a formidable and yet eminently watchable way. "I can't imagine it. But I can imagine how stupefyingly boring NBC's would be if they did do it." He's probably thinking of those stuffy old three-hour NBC "White Papers," which NBC News President Bill Small now says will be 90 minutes tops.

As for ABC, they have tackled Armageddon and other such subjects, but Stringer is not enamored of ABC's "1-2-3 fast-cut stuff" or an inferred philosophy of "never let the audience do the work" and "never let any interview go longer than three seconds." At CBS News, they know they are the best. Sometimes it just makes them insufferable. At other times, it helps them produce blockbusters like "The Defense of the United States."

Is five hours too much to spend on this subject? And might the explicit and implicit alarmism of the report, especially the segments on America's apparent lack of preparedness and military competence, contribute to a growing trend back to brinkmanship and a good guy/bad guy world view? How people will react to the series cannot be predicted, but "Defense" could become the most watched, and most talked-about, documentary ever shown on television.

For a report that aspires to be comprehensive, "Defense" is marred by some curious omissions. Little or nothing is said about the ugly potential for warfare in outer space or about the more imminent specter of the neutron bomb. The MX missile gets short shrift for such a volatile topic. These are relatively minor blemishes.

Beyond the major points made with word and picture, the five hours are punctuated with telling, mordant or darkly comic details. "We believe we do" have the capacity to destroy the Soviet Union, says retired Adm. Powell Carter -- but not completely . B-52s armed with nuclear warheads can take off with the drapes closed to prevent the pilot's going flash-blind from a nuclear explosion. In event of attack, the president would be taken to a getaway plane called Kneecap, kept ready night and day and guarded by a soldier ordered to shoot anyone who crosses the red line surrounding the plane.

A woman who lives near SAC headquarters in Omaha and has comtemplated nuclear attack says that in such a case, "There's no place to go. You don't do anything. You say a prayer and that's all." During a test of the Pershing missile in Europe, it is observed that in the event of an attack, soldiers might die without gas masks but that if they were wearing them, the intense heat would melt them to their faces, and kill them. At a hardware show given by defense contractors, weapons are hawked as merrily and innocently as new cars or stereo equipment.

In an ingenuous attempt to get information from the Soviets, Cronkite is seen mailing a letter to Brezhnev at a Moscow post box. But he never gets an answer.

And twice during the five hours, in scenes filmed at U.S. military installations where troops are training for the dread possibility of World War III, we see a banner on a wall displaying a hammer and sickle and the words, "The Threat." Ah yes, The Threat. If only it were that simple. "The Defense of the United States" raises some of the most important and essential questions it is possible to raise, but the people who made it are smart enough to know they hardly found any of the answers. CAPTION: Picture 1, A composite of scenes from "The Defense of the United States": Dan Rather interviewing Secretary of Defense Caspar W. Weinberger, and $87,000 worth of special effects sets off an atomic blast at Omaha, Neb. The Washington Post from CBS News photos; Picture 2, Walter Cronkite in Russia.; Picture 3, Bob Schieffer with a MIRV.