Even scholars of U.S. foreign relations who never met David Halberstam, but came of age in the 1960s and after, felt a sense of loss at his passing in a car accident last spring. As a young New York Times correspondent early in that decade he became one of the first voices of dissent over the American conduct of the war in Vietnam. Then, in 1972, he produced “The Best and the Brightest,” a captivating portrayal of the folks who embroiled America in that tragic conflict. To those of us who had chosen to study American-East Asian relations as a means of understanding how our country got into that war, Halberstam stood tall.

“The Best and the Brightest” was a hard act to follow, although Halberstam went on to write numerous other books, on topics ranging from high politics to newspapers and sports heroes. A master at capsule biography and historical digression, if not brevity or integrated narrative, Halberstam consistently provided penetrating insights into personalities and relationships, and he unfailingly viewed those in power with a critical eye. His final book, “The Coldest Winter” (Hyperion, 736 pages, $35), on the early stages of the Korean War, is no different.

That conflict is often referred to as “forgotten” in America, but now, over a half-century in the past, it has generated a large body of archival sources and scholarship. Halberstam demonstrates mastery of the oral histories of participants, but is less comprehensive in his use of unpublished written materials from the period. He displays a wide-ranging but selective grasp of recent scholarship, a tendency that produces some missteps, especially in coverage of the non-American sides of the war.

The list of missteps is fairly long. Most South Korean soldiers did not go into battle “almost completely untrained” and, although their early performance left much to be desired, their efforts were essential to the defense of the Pusan Perimeter. Late in the war, South Korean units manned 70% of the front lines. On Sino-Soviet relations, Chinese leader Mao Zedong’s visit to Moscow in late 1949 and early 1950 was far from being in “all ways a disaster.” If Mao made concessions that he resented to his Soviet counterpart Joseph Stalin, he also achieved the military alliance he sought, as well as $300 million in assistance. On October 2, 1950, Mao did not, as Halberstam writes, send a telegram to Stalin saying China would dispatch 12 army divisions to Korea. He wrote such a message, but the one he sent stated that China had decided for the moment not to intervene in Korea.

Halberstam is also shaky on some details of Korean geography and even on American leadership in Washington. He places Seoul, the South Korean capital, 60 miles below the 38th parallel (it’s about 30), and the Pyongyang-Wonsan line 300 miles from the Yalu River, the northern border of most of North Korea (at no point is it that far away and at points it is under 150 miles away). Back in America, William J. Donovan, the World War II leader of the Office of Strategic Services, was never head of the Central Intelligence Agency, contrary to Halberstam’s account, and George F. Kennan, the author of American containment strategy, did not have “great knowledge” of China or even consistent wisdom regarding Japan and Germany. Halberstam bungles his description of the crucial instructions of the Joint Chiefs of Staff to General Douglas MacArthur, the commander of American and Allied Forces in Korea, which were to guide his operations in North Korea. He was not, as Halberstam claims, “to break off contact if his troops met with either Russian or Chinese forces.” In fact, the instructions said nothing about what MacArthur was to do if he confronted Soviet forces in North Korea and if he confronted Chinese forces he was to “continue the action as long as, in your judgment, action by forces now under your control offers a reasonable chance of success.”

Such mistakes irritate the careful scholar, yet in a way they are secondary quibbles. First and foremost, Halberstam’s book is about the experience of American soldiers in Korea and the impact on their fates of American leadership in Tokyo and Washington, and in that purpose it succeeds admirably. American leaders failed to anticipate North Korea’s attack on South Korea in June 1950 and to prepare their military for that or virtually any other eventuality. Later in the year they failed to prepare for Chinese intervention in Korea. The people who bore the greatest sacrifice for these failures were the young men of the U.S. armed forces. While Halberstam provides balanced assessments of the key figures in Washington, skillfully delineating the domestic pressures that sometimes produced bad decisions, he is merciless toward the egomaniacal General MacArthur, General Charles Willoughby, his sycophantic chief of intelligence, and General Ned Almond, the arrogant and racist commander of his X Corps. Some scholars will dispute Halberstam here: I will not be among them. If less than definitive, the weight of the evidence clearly warrants his harsh judgments.

The most original research in “The Coldest Winter,” though, is on Americans in the line of fire in Korea, from the lowly privates and noncommissioned officers up through the battalion and regimental commanders, especially in the American Second Division. Halberstam conducted dozens of interviews with veterans of the first eight months of the war, and the stories of combat that he weaves together from these and other sources are the most compelling in the book. Among the numerous characters are Colonel Paul Freeman, a West Pointer who commanded the 23rd Regiment with bravery and skill through numerous trying circumstances only to be relieved from command in the middle of one of the decisive battles of the war; Sam Mace, a West Virginian with a fourth-grade education who, after distinguished combat in the Battle of the Bulge in World War II and the Pusan Perimeter in Korea, was promoted to second lieutenant because his company commander thought him “quite possibly the best soldier he had ever served with,” and Lieutenant Gene Takahashi, a Japanese-American who joined the army in 1945 to escape an internment camp and by November 1950 commanded a platoon in Love Company, an all-black unit that got caught in what became known as the Gauntlet, a narrow stretch of road in northwestern Korea where the Chinese surprised and overwhelmed American and Allied Forces. Only 10 of the 170 men of the company made it through. In his account of the battle experience, Halberstam’s prose is lively but his detailed descriptions of some battles will strain the patience of all but the veterans who endured them and the most dedicated military buffs.

Yet patience is essential if we are to fully grasp Halberstam’s message, hardly original but always warranting repetition: that the decisions of political and military leaders have life-and-death consequences for the men and women of the armed forces, making it essential to hold those leaders to a high standard — not only in their own time but before the court of history.

Mr. Stueck is a professor of history at the University of Georgia and the author of several books on the Korean War.