The U.S. Army has developed doctrine over centuries to guide how it organizes, trains, and operates in war. The book by Walter E. Kretchik shows that Army leaders have been forward-thinking and adaptable. The history of Army doctrine is divided into four eras, each shaped by different factors. Doctrine has influenced the Army's performance, but has sometimes failed to predict future wars accurately. The book highlights key figures who shaped doctrine in each era, and how it was applied in practice. Doctrine is still crucial in Army training, providing a common framework for all soldiers.