FOR MORE than 30 years now the Korean War has been as obscure to most Americans as "Fifty-Four Forty or Fight" or Pershing's expedition against Pancho Villa. Americans approved of it, for a while, in their minds if not their hearts, but they quickly turned against it and even more quickly forgot it.

Now comes Donald Knox's gem of an oral history to remind us that the Korean War was lethal enough to satisfy even the most bloodthirsty. About 55,000 Americans died in three years in Korea -- about as many as were killed in our eight-year military involvement in Vietnam -- and more than 100,000 were wounded.

As the subtitle indicates, The Korean War: Pusan to Chosin covers the first six months of the conflict -- from the North Korean invasion of South Korea on June 25, 1950, until the end of the year when the United Nations forces were regrouping after their disastrous defeat at the hands of the Chinese Communists at the Chosin Reservoir and along the Chongchon River near the Manchurian border.

Knox, a television producer, has put an enormous amount of work into this account, and it shows. The book consists primarily of interviews with surviving Marines and soldiers laced with excerpts of operational orders and unit histories and his terse narrative. He cuts the interviews up and arranges them chronologically; the result, aided by helpful maps, is both a clear overview of the first six months' fighting and vivid accounts of infantry warfare from the rifleman's point of view.

At 4 a.m. June 25, seven divisions (of about 10,000 to 12,000 men each) and five brigades of North Koreans supported by Soviet-built tanks and aircraft crossed the 38th Parallel, the dividing line between the Soviet and U.S. occupation forces in 1945 that became the de facto boundary that split the Hermit Kingdom into North and South Korea. They quickly captured Seoul, the South Korean capital, and by the first of August had backed the South Korean and U.S. forces into a 200-square mile area around the southeastern port of Pusan known as the "Pusan Perimeter."

For six weeks the U.N. forces held on and built up their strength, then with the daring amphibious invasion of Inchon on September 15 began the breakout and destruction of the North Korean army. Hoping to stabilize the entire peninsula, they pushed north of the 38th Parallel and almost to the Yalu River, the 8th Army in the west, X Corps in the east. Then, in late November they were ambushed by at least 300,000 Chinese who had infiltrated by night and hid out in the hills by day, completely undetected by U.S. intelligence.

The Korean peninsula is about the size of Great Britain, and if there is a worse place to fight a war it doesn't immediately jump to mind. It is mostly mountainous and hilly and the temperatures range from 105 humid degrees in summer, with a monsoon season that turns it into a quagmire, to 40 below in the winter with bitter Siberian winds howling down from the north. In August the U.N. forces lost almost as many casualties to the heat as to gunfire, and in December the hidden enemy was frostbite.

The book is eloquent tribute to the inexorable, merciless attrition of infantrymen in combat.

On September 3, in the second battle of the Naktong Bulge, part of the desperate fight for the Pusan perimeter, 2nd Lt. Frank Muetzel, a platton leader in Able Company, 5th Marines, led an assault on a hill that was part of Obongni Ridge, or "Bloody Ridge" as it was called. The attack was textbook-perfect with supporting fire from tanks, aircraft and machine guns, but two of Muetzel's rifelmen crossed the crest of the hill and were killed by a sniper, as was a third Marine who tried to rescue them.

"I was in tears," Muetzel recalls. "Even when we attacked a hill properly, it was still sheer murder."

On September 24, seven weeks after the Marines arrived in Korea and a week after the Inchon landing, Pfc. Doug Koch of Dog Company, 5th Marines, was hit in the legs and evacuated. He was the last original member of his platoon (44 men at full strength). In seven weeks it had taken 100 percent casualties -- more than 100 percent because some replacements had already been casualties.

The U.S. Army occupation troops in Japan that were first fed into the meat grinder for the most part had enlisted with little thought of fighting a war and were enjoying the good peacetime life. Reading this book in the context of a relatively large standing army is a vivid reminder of the difficulty of maintaining esprit, discipline and training in non-elite units and of how important these factors are for troops who may be hurled unexpectedly into combat at any moment.

From the Korean vets' accounts, it is apparent that a substantial case could have been made for court-martialing the U.S. Army's entire Far East chain of command, from Douglas MacArthur on down, for dereliction of duty, assuming that the primary duty of a military command is to train, motivate, condition and equip its troops for combat.

Although many fought courageously and well, it is clear from these accounts that thousand of Army troops became casualties or were captured in the early fighting through lack of training and unit cohesion.

The Marines, by contrast, took fearsome casualties in some of their encounters, but they were carrying out their mission. Their discipline and tactics -- such as securing the high ground on either side of the road out of the Chosin Reservoir -- were the keys to their escape.

In its agonizing and harrowing withdrawal from Chosin Reservoir, the First Marine Divisionht out its dead, wounded and heavy equipment and the demoralized survivors of an Army regimental combat team, while inflicting grievous losses -- the equivalent of more than three full divisions -- on a communist field army that outnumbered it by about 8-to-1. This was an operation marked by valor, discipline and professionalism that ranks with Iwo Jima, Bastogne and Belleau Wood in American military annals, and the Marines suffered their highest casualty ratio ever. The 8th Army withdrawal also was professional and generally in good order.

Nevertheless, when he assumed command of the 8th Army at Christmastime, Lt. Gen. Matthew B. Ridgway, the former paratroop commander, expressed dismay over "how road-bound this army was, how often it forgot to seize the high ground along its route . . . how reluctant it was to get off its bloody wheels and put shoe leather to earth . . . "

In the ensuing months Ridgway and his successor, James Van Fleet, reformed the 8th Army into a formidable force and it was well they did. Two-and-a-half years of a murderous war of stalemate and attrition reminiscent of World War I lay ahead before the signing of the armistice.