Throughout the 19th century the U.S. Army alternated between a small regular force scattered at isolated forts and large forces built quickly to fight major wars. In his book Preparing for War, Dr. J.P. Clark breaks the officer corps down into four generations between 1815-1917 whose ideas about professionalism and how to prepare for war were shaped by their institutions, experiences and culture. Clark is an active duty military officer who taught history at West Point and served as a strategic advisor at the Pentagon and British Ministry of Defense.