This fascinating true story about war, intrigue, defection to the enemy, and brutal military justice is a dramatic example of the conflicts that frequently arise between humanitarian values and inflexible military regulations.

Shamrock and Sword's setting is the U.S.-Mexican War, remembered by Americans as an illustration of Manifest Destiny, the inevitable extension of the American frontier. It is remembered differently by Mexicans, who lost a substantial portion of their territory to an invading army. Perceptions on both sides of the border will be reshaped by Robert Ryal Miller's account of American soldiers who deserted to fight in the Mexican army. Miller uncovers the reasons for these desertions, presenting the soldiers' stories as they are revealed in records of the time. Many of these deserters were immigrant Irishmen.

Contrary to what has been supposed, however, the Saint Patrick's Battalion included men of a dozen nationalities. Choosing for different reasons to fight under the Mexican flag, all were treated as deserters, and those captured were court-martialed by the U.S. Army. Fifty were executed; others were whipped and branded. The leader of the group, John Riley, was branded with a D on both cheeks. The Mexican government, on the other hand, viewed the men of the Saint Patrick's Battalion as heroes, awarding them honors and erecting a monument to them.