This previously unpublished diary records the intuitive reflections of an educated man who served as a military administrator in northern Mexico during the Mexican War. Colonel Samuel Ryan Curtis, engineer, lawyer and graduate of West Point, arrived in Mexico in July of 1846 as commander of the 3rd Ohio Volunteer Regiment to find a volatile and chaotic situation in occupied towns along the Rio Grande. American civilians of the lowest sort--men and women--mingled with Mexican townspeople, robbing, murdering, and raping. Neither civil nor military law made provisions for governing municipalities under such conditions. Nor was the U.S. military prepared for a guerrilla struggle against renegade Mexican soldiers and bandits. This is Curtis's story of maintaining order under nearly impossible conditions--of death and suffering in his regiment from disease, not fighting, and of the tedium of army camp life. Curtis was a keen observer who chronicled martial unrest and political conditions in Matamoros, Camargo, and Saltillo. He also documented social and economic circumstances, as well as flora and fauna, and even the weather.