Samuel Lyman Atwood (Slam) Marshall, military historian, was born on July 18, 1900, in Catskill, New York, son of Caleb C. and Alice Medora (Beeman) Marshall. A few years after his birth the family moved to Boulder, Colorado, and subsequently to Niles, California, where young Marshall worked for a while part-time as a child actor with the Essanay Motion Picture Company. The family settled in El Paso in 1915. Marshall was attending El Paso High School when the United States went to war with Germany, and on November 28, 1917, he enlisted in the army. He served in France as a sergeant with Company A, 315th Engineers, of the Ninetieth Division. In later years he claimed that he had received a battlefield commission, commanded troops on the Western Front, and was the youngest second lieutenant in the American Expeditionary Forces. It has since been shown, however, that he was not commissioned until 1919, after he had completed the course of instruction for infantry-officer candidates at La Valbonne. Thereafter he held various posts with the Services of Supply until he left France in the summer of that year.
After returning to civil life in September 1919, Marshall briefly attended the Texas School of Mines (now the University of Texas at El Paso) and then worked at various jobs until 1922, when he began his journalistic career with the El Paso Herald. During this time he adopted his nickname, Slam, an acronym of his initials, for his bylines. He served as a reporter, sports editor, and city editor of the paper until 1927, when he joined the staff of the Detroit News. There he remained as editorial writer for most of the remainder of his life except for absences connected with military service. During the pre-World War II years he traveled frequently to Mexico and Central America as a correspondent covering events in those countries. He also became a commentator on military affairs, and when the war broke out in 1939 he began an evening radio broadcast on station WWJ. In 1940 he published his first book, Blitzkrieg, which was favorably received by the great British military historians J. F. C. Fuller and Basil H. Liddell Hart.
Marshall reentered the army as a major in September 1942. He was assigned initially to the Information Branch, Special Service Division, Services of Supply, at the War Department. In 1943, by then a lieutenant colonel, he was transferred to the newly formed Historical Division of the General Staff, where his first task was to write a definitive analysis of the recent raid of James H. Doolittle on Tokyo. In October 1943 he was sent to the central Pacific to develop methods of combat research. In pursuing this work, he was attached to the Twenty-seventh Infantry Division during the invasion of Makin Atoll in the Gilbert Islands and to the Seventh at the taking of Kwajalein. In these operations he devised the after-action, group-interview technique as a means of determining precisely what had happened in an engagement and why success or failure ensued.
In June 1944 Marshall was sent to the European Theater of Operations on temporary duty. He was attached to the Historical Section, Headquarters ETOUSA, and remained with that organization until the end of the war. In Europe he introduced the group-interview technique that he had developed in the Pacific and applied it to numerous major operations, including the D-Day airborne landings and the Ardennes campaign. In July 1945 he was designated theater historian, and a year later he returned to civilian life and the Detroit News. In 1947 Marshall published what is probably his most influential work, Men Against Fire. Based on his after-action interviews during the World War II, he advanced the claim that fewer than a quarter of American infantrymen actually fired their weapons in any given action. The work stressed the importance of training, discipline, and, above all, communication, in overcoming the paralyzing effect of the modern battlefield. The book was discussed in army circles and had some impact on the formation of military doctrine in the immediate postwar period, yet some high-ranking officers expressed doubt about the source and veracity of his data, and by the 1980s several articles by professional historians had appeared questioning his research methods.
In the postwar years Marshall, a colonel in the Officers' Reserve Corps, continued his close association with the army. He lectured frequently at service schools throughout the country and served occasional brief periods of active duty at the Pentagon. During the Korean War he served in the winter of 1950–51 as an operations analyst for the Operations Research Office, a Johns Hopkins University think tank under contract to the Department of the Army. In 1953 he again visited Korea as a journalist.
He was appointed a brigadier general in the United States Army Reserve on May 7, 1957, and on July 31, 1960, he was placed on the retired list. Even in retirement he continued his endeavors on behalf of the army. Beginning in 1966 he made a series of quasi-official visits to Vietnam. Although his status was essentially that of a private individual, he traveled with the army's blessing and cooperation and worked in the army's interest, employing once again his group-interview technique, imparting it to a new generation of army historians, and making known through his writings the army's point of view in that unpopular war.
In his long career as a military commentator, Marshall wrote more than thirty books, dozens of journal articles, and countless newspaper, radio, and television pieces. His books include The River and the Gauntlet (1953), Pork Chop Hill (1956), Battle at Best (1964), and Battle in the Monsoon (1967). In addition to his participation in four American wars, he observed the Sinai War of 1956 and the Six-Day War of 1967, the Lebanon crisis in 1958, the civil war in the Congo in 1961, and the unrest in Southwest Africa in 1965. Marshall received many honors and awards during his lifetime, including the Legion of Merit, the Bronze Star with oak-leaf cluster, the Combat Infantry Badge, and the Legion of Honor. He was also elected to the National Cowboy Hall of Fame in 1972.
In 1974 he returned to El Paso to live in retirement amidst the scenes of his youth. He marked the occasion by donating his professional library to his alma mater, the University of Texas at El Paso, where it now occupies a separate room in the University Library. Marshall died on December 17, 1977, and was buried in Fort Bliss National Cemetery with all the honors due a departed general officer. His first marriage, to Ruth Elstner of El Paso, ended in divorce. They had one son. His second wife was Edith Ives Westervelt of Detroit, who died of multiple sclerosis in 1953. Marshall had three daughters with his third wife, Catherine (Finnerty), who survived him by eleven years. His autobiography, Bringing Up the Rear, was published in 1979.