Shamrock and Sword is the story of the St. Patrick's Battalion (San Patricios), which was composed primarily of American soldiers who chose to fight under the Mexican flag in the U.S.-Mexican War. More than two hundred American deserters (many of them Irish immigrants) joined the Mexican army to fight against their former countrymen.

"Miller thoroughly investigates every legend about the battalion and the individuals who served in it. . . . Miller's book is both a detailed history of the San Patricios and a good brief summary of the entire U.S.-Mexican War."

-Journal of Arizona History

Robert Ryal Miller was Professor Emeritus of History at California State University, Hayward. He was the author of Mexico: A History and Juan Alvarado: Governor of California, 1836-1842, also published by the University of Oklahoma Press.