In the opening campaign of the Korean War, the First Provisional Marine Brigade participated in a massive effort by United States and South Korean forces in 1950 to turn back the North Korean invasion of the Republic of Korea. The brigade's actions loom large in marine lore. According to most accounts, traditional Marine Corps discipline, training, and fighting spirit saved the day as the marines rescued an unprepared U.S. Eighth Army, which had been pushed back to the ""Pusan Perimeter"" at the southeastern tip of the Korean peninsula. Historian and retired marine Kenneth W. Estes undertakes a fresh investigation of the marines' and Eighth Army's fight for Pusan. Into the Breach at Pusan corrects discrepancies in earlier works (including the official histories) to offer a detailed account of the campaign and place it in historical context.

Drawing on combat records, command reports, and biographical materials, Estes describes the mobilization, organization, and operations of First Brigade during the first three months of American participation in the Korean War. Focusing on the battalions, companies, and platoons that faced the hardened soldiers of the North Korean army, he brings the reader directly to the battlefield. The story he reveals there, woven with the voices of soldiers and officers, is one of cooperation rather than interservice rivalry. At the same time, he clarifies differences in the organizational cultures of the U.S. Army and the Marine Corps.

Into the Breach at Pusan is scrupulously fair to both the army and the marines. Estes sets the record straight in crediting the Eighth Army with saving itself during the Pusan Perimeter campaign, but he also affirms that the army's suffering would have been much greater without the crucial, timely performance of the First Provisional Marine Brigade.