If you think the past 40 years have flown by, wait until you see them fly by again tonight on ABC. "45/85," a sweeping three-hour retrospective on the four decades since World War II, reveals the global resources of ABC News cranked up to whatever comes right after "full throttle."

Interweaving dozens of interview subjects with retrieved archival footage, newsreels, previously untelevised color home movies, and skillfully assembled impressionistic montages of everyday life during the decades in question, "45/85" achieves the rarely realized status of epic documentary. It is by far the best of the three-hour maxidocs undertaken by ABC News. One of its chief achievements lies in the fact that its mammothness does not render it innocuous or amorphous.

A nearly indescribably lively history lesson that is also a breakneck exercise in high-tech teleportation, "45/85" has a depth to match its breadth, and that is primarily because the report, which occupies all of prime time on Channel 7 starting at 8 tonight, proceeds from a point of view. It is not going to attempt to recapture every nuance of political and social life in the past 40 years, but instead size things up in relationship to what coanchor Ted Koppel, in the program's opening, calls "the most important" single continuing story: the "ongoing confrontation" between the United States and the Soviet Union.

This "main focus" accommodates a great many related subplots along the way. But some viewers, attracted by the title and the implicit promise of a comprehensive, CinemaScopic portrait, may be alarmed to see so little attention given, say, the Civil Rights struggle as the years go charging by. Viewers who stay with the broadcast, on the other hand, are likely to feel ABC News has made its case persuasively; this was the right approach, this is the story that has affected nearly all other stories.

To keep it a story of people and not just of issues and places and implacable forces, executive producer Av Westin, perhaps with a bow to Warren Beatty's movie "Reds," includes the filmed reminiscences of "witnesses" who participated in history as it was made. Some only speak a sentence or two (and not every one of those sentences is illuminating), but the admirable thing about the technique is that we hear not just from the celebrated and the notorious, but also from the relatively anonymous, people who got caught up in one piece of tumult or another and whose first-hand observations have an immediacy that the recollections of statesmen sometimes lack.

There are plenty of statesmen. All three living ex-presidents were interviewed (Richard Nixon, not surprisingly, gets the most air time), as well as Henry Kissinger, Dean Rusk, Ted Sorensen, Robert McNamara, Pierre Salinger, Clark Clifford, Alexander Haig, Cyrus Vance, Andrew Young, Zbigniew Brzezinski and Jeane Kirkpatrick. They are joined by international voices ranging from Le Duc Tho, negotiator for North Vietnam, to Abba Eban, former Israeli foreign minister, to Kenneth Kaunda, president of Zambia, to Sir John Colville, private secretary to Churchill, to Fyodor Burlatsky, speechwriter to Khrushchev.

Akio Morita, chairman and cofounder of Sony Corp., traces the origins of his company, and of Japan's economic rebirth, to a dinner he attended in Germany and to a little paper umbrella on a dish of ice cream; he was chagrined that this paper umbrella was then representative of his country's level of industrialization. Aviation entrepreneur Freddie Laker, it turns out, was a pilot during the Berlin Airlift, and recalls the perils of those flights. Longtime Kennedy aide, now Kennedy librarian, Dave Powers does his impression of JFK on the night he ate chicken with him at the White House during the Cuban Missile Crisis and wondered if it was to be "The Last Supper" -- ever. And George McGovern remembers meeting LBJ after he left the White House and returned to Texas and discovering the president had grown "shoulder-length hair" just like that sported by the rebellious youths who had been his nemeses.

Alert producers intersperse these anecdotes with observations by people we may remember only as fleeting images: Marvin Kingsbury, the smooching sailor in the iconographic Alfred Eisenstaedt VJ-Day photograph; Mario Cabello, now a lawyer, once a member of the very ill-fated Bay of Pigs invasion force ("We were sitting ducks"); and Kim Phuc, grown into womanhood today, but in 1968 a little girl on fire in a memorably horrifying photograph made on a Vietnam road after a napalm attack.

A diplomat stationed in Saigon grimly recalls what he said aloud as a helicopter carried him and others to safety after the city fell, and as he looked down on the hordes besieging the embassy: "There they are, the hearts and minds that we won."

An interview with President Reagan, taped Monday in the White House by Koppel and program coanchor Peter Jennings, was not available for preview. The 20 minutes taped will be edited down to eight.

Once the broadcast is over tonight, it is not really over. Its topics and themes will be picked up for discussion on a special 90-minute edition of "ABC News Nightline," with Koppel's guests including Alistair Cooke, Theodore H. White and Kirkpatrick, as well as a seasoned Kremlin "Americanologist." There are few conspicuous absentees, meanwhile, in the cast of nearly thousands to be seen on "45/85," but among them one would have to list Sen. Edward M. Kennedy (D-Mass.) and ABC's own expensively hired gun George F. Will, whom one would expect to encounter in any program that deals so extensively with the Red Menace.

And it is a menace, isn't it? Throughout the program, one is haunted by the unanswerable question: What might the world and its societies have accomplished since the war's end if you could subtract the Soviet Union and its dogged, dispiriting aggression from the scenario? Of course, another abiding theme of the program is American error. The pageant of fatal misjudgments began very early, according to the documentary, when American military bases around the world were prematurely shut down as part of postwar euphoria. Nixon recalls that at the time, many thought Churchill "too belligerent" in his famous "Iron Curtain" speech. "However," Nixon says, "he proved to be right."

The litany of mistakes does not bolster one's faith in American leadership, past or, implicitly, present. Failure to recognize the People's Republic of China at the propitious moment of 1949, failure to anticipate Chinese participation in the Korean war, failure to assist Hungary when the Soviets brutally crushed rebellion there, failure to come clean immediately on the disastrous U-2 mission of Francis Gary Powers (an event re-created through audio memoirs left by both Eisenhower and Khrushchev), failure to perceive the hopelessness of involvement in Vietnam (for which JFK gets much of the rap in this report) -- "45/85" puts all these failures together. It is not a petty picture.

Jennings, his delivery as usual too self-consciously contemplative, and Koppel, who has far more high-impact punch, narrate parts of the report from a New York studio, but logged a sizable number of air miles, too. Koppel turns up in Times Square, Missouri, Istanbul, Jerusalem, Saigon and Useppa Island, Fla., once the CIA training ground for the Bay of Pigs. Jennings materializes in Panmunjon, Tokyo, Suez, West Berlin, Washington and Cairo, among other locales.

As fascinating as it is ambitious, "45/85" seems especially indispensable viewing for members of the baby boom generation. These are the years of their lives, best and worst. From the giddy bravado of a late-'50s commercial ("Introducing the American way of life on the threshold of the golden '60s!") to the glum reality of Carter-age malaise ("You just don't know what tomorrow is going to be like; it scares you," a woman says), one can trace the fate of the common spirit that Ronald Reagan appears now to have lifted, though the suspicions remain that he's done it with mirrors. Ads for "45/85" invite a viewer to see "where the future began"; the question it asks is whether any of us will ever believe in the future again.