In a monastery built by Franciscan monks on the site of an Aztec war god's temple, a rogue battalion of cannoneers prepares to make its final stand along a strategic road to Mexico City. Reviled by its enemies and lionized by its allies, Saint Patrick's Battalion will fight under an emerald green flag with the fury of the proud and the damned. And as James Alexander Thom's extraordinary new novel reaches a shattering climax, the reader hurtles into a collision between true loyalty and true betrayal - and between the best and basest reasons for war.

Narrated by two soldiers on opposite sides of the Mexican-American War of 1846, Saint Patrick's Battalion tells the true but little-known story of an Irish immigrant who deserted from the U.S. Army and was joined across enemy lines by hundreds of comrades. Driven by the abuses of Protestant West Point—trained officers and the realization that they were attacking fellow Catholics, John Riley and his San Patricios abandoned their adopted country and took their place proudly alongside the dashing Hidalgo horsemen and stolid native Indians who were being used by the Mexican army as cannon fodder against the foreign invaders. Though hopelessly misled by the vainglorious Santa Ana, and facing such future military legends as a brooding young Ulysses S. Grant and the brilliant captain Robert E. Lee, Riley and his fighters were responsible for an enormous number of American casualties - and would eventually pay a brutal price for their treachery.

Its narrative foreshadowing America's Civil War, Saint Patrick's Battalion asks haunting questions about American expansionism, racism, and the machinations of a war that began before it was declared. From horrific depictions of cannonade warfare to the quiet corners of doubt, courage, and love in men's hearts and minds, James Alexander Thom's novel takes us on an astounding adventure into beautiful, harsh Mexico - and dramatically chronicles a crucial, bloody chapter in the making of America.