They were proud and doomed, Irish rogue cannoneers under a green silk banner, fighting against their former comrades-in-arms to defend Catholic Mexico against the invading U.S. Army.

Their choices were to win, die in battle, or hang as deserters. To the Mexicans they were heroic saviors, but they were seen as despicable traitors by the West Point officers who faced their grapeshot and cannonballs on every major battlefield Matamoros to Mexico City.

Survivors of the Irish battalion's deadly gunnery would later lead armies against each other in Civil War, many of them - Robert E. Lee, Stonewall Jackson, Braxton Bragg - ironically deemed the same sort of traitors. This astonishing true tale from an almost forgotten war is told through the eyes of two boys who know and admire the idealistic Irish leader John Riley: an Army camp errand boy who keeps a diary, and a Mexican military school cadet whose widowed mother becomes the Irishman's tragic lover.