FOR MANY YEARS millions of Americans have been consumed by passions let loose by our role in Vietnam--a conflict that should be known historically as the Second Indochina War--but few among us realize how close the United States came to plunging into the First Indochina War, and quite possibly with the use of American atomic bombs.

The Sky Would Fall will surely be the definitive account of that earlier almost-but- not-quite intervention. It is an instructive story, full of lessons to contemplate nowadays as the United States extends its military reach to such volatile areas as the Middle East and Central America. That lessons are not, however, easily learned is evident in the fact that having escaped the First Indochina War we plunged, as author John Prados suggests, "eyeless into Indochina" a few years later.

Indochina--present-day Vietnam, Laos and Cambodia (or Kampuchea)--was a pre-World War II French colony that fell to Japan. Charles de Gaulle was determined to get it back and France did so, but by 1945 Ho Chi Minh's Communists had captured much of the region's political soul. Thus the French from 1946 fought the First Indochina War with Ho's Viet Minh until Paris abruptly pulled out in the wake of the 1954 Geneva agreement on a "temporary" division of Vietnam.

The Sky Would Fall is the story of the dramatic weeks in spring 1954 when a weak French government, its troops trapped in a valley called Dien Bien Phu, appealed to its ally America to rush to its rescue. At the moment American attention was chiefly riveted on a senator from Wisconsin named Joseph McCarthy whose career was then reaching a climactic moment. And it must not be forgotten that less than a year earlier the Korean War had ended in an unhappy truce, that most Americans believed the Soviet Union was behind that war in which "Red China" had fought against us, and that the common Washington view was that the truce allowed the Communists to switch their attention and their forces to Indochina.

President Eisenhower, as early as 1951, Prados shows, accepted the "domino theory" which postulated that if "Indochina falls to (the) Commies, it is easily possible that the countries of Southeast Asia and Indonesia would go, soon to be followed by India." Essentially the same arguments were made to justify the American role in the Second Indochina War.

The basic story of the American role in the first war became known by mid-1954 and this reviewer, then a Washington Post foreign affairs reporter, spent a vast amount of time tracking it down both in Europe and in Washington and writing about it. The Eisenhower administration never admitted how close it came to intervention, though Secretary of State John Foster Dulles confirmed the essence of such a policy in his famous "brink of war" interview in Life magazine.

What John Prados, a 32-year-old historian and creator of simulated games, has done is to flesh out the story with a lot of new material from recently declassified documents, including Ike's diary and Dulles' notes on phone conversations with the president. Prados writes dispassionately but nonetheless catches the flavor as well as the facts of the internal American debate over whether or not to try to rescue the French.

The core of the story revolves around Admiral Arthur Radford, chairman of the Joint Chiefs of Staff and a Cold Warrior if there ever was one, and his efforts--with varying degrees of cooperation from Dulles--to create "Operation Vulture." This was the plan to use nuclear weapons in Indochina under some circumstances. "Raddy" discussed with the French using aircraft from the carriers Essex and Wasp, which he had moved into the Gulf of Tonkin to photograph airfields in South China, and B-29 bombers from the Philippines to prevent "the loss of all S.E. Asia to Communist domination," as he put it to Ike. By then the United States already was paying the bulk of the war's financial cost to the French and supplying much of their weaponry.

Operation Vulture led Eisenhower to list four conditions for American action: a French request, a French promise of some form of independence for the three Indochina states, the assent of Congress and the participation of allies, especially the British. When Dulles secretly broached intervention to congressional leaders, they--most notably and ironically, Lyndon Johnson-- stressed the need for allies. Because the British refused to take part, Congress was never asked. British Foreign Secretary Anthony Eden, as noted in this book, told me in Geneva when I asked him who had stopped intervention: "I guess I did." He had no trouble winning Prime Minister Churchill's assent.

To me, Prados is convincing in his evidence that Eisenhower would have taken the plunge had his conditions been met; and I have had no doubt, then or now, that Congress would have agreed with a forceful request from Ike. Vice President Nixon had let Americans know that the United States might very well have to "dispatch forces" to prevent "further retreat" in Asia. But General Matthew Ridgway, Army chief of staff, objected privately to Ike, which helped convince the president, eventually, that air strikes would indubitably lead to ground intervention for which the United States was ill prepared.

Less certain is the question of using nuclear weapons. The Americans and the French both concluded they would be inappropriate, or worse, in any effort to save Dien Bien Phu. But Radford's plans included their use against the Chinese, should they intervene after the United States did, and with this Ike concurred. Prados offers new and convincing evidence that Ike at the National Security Council table even considered "loaning" atomic bombs to the French. French Foreign Minister Georges Bidault insisted that Dulles, just before the Geneva conference opened during the last agony of the French garrison at Dien Bien Phu, "ask(ed) me if we would like the U.S. to give us two atomic bombs," as he wrote in his 1965 autobiography.

Prados' conclusion, which I believe is correct, is that "whether or not Secretary of State Dulles made a specific offer of the A-bombs on April 23, it is true that the Americans did say something that at least could have been construed as such." Dulles never conceded making any such offer, blaming it all on the high degree of confusion at the moment and on Bidault's own state of near collapse.

In the end, although Eisenhower demurred after the British refusal and Ridgway's objections, he concurred when Dulles, a year later, decided that South Vietnam could be "saved." Thus began that fateful American march into the quagmire, despite the lessons from the First Indochina War.