“Changing Flags” is the true story of John Riley, a cocky Irish professional soldier who deserts the U.S. Army to fight for Mexico at first for money, then for something loftier in the Mexican War of 1846-48. With gritty action, romance, and humor, it brings to life his torturous journey of reformation and that of rowdy Irish and German fellow deserters who follow him in pursuit of a shared dream. Their epic tale begins in Michigan when Riley launches a plan to afford passage home to Ireland: He enlists intending to desert in Texas, where U.S. troops are being sent to end a border dispute with Mexico. A former British soldier, he anticipates becoming sergeant with higher pay. Denied promotion, he feels trapped across the Rio Grande from Matamoros. Then he is slipped a flyer offering cash, rank, and land to U.S. soldiers who join the Mexican army. That night, Riley crosses the moonlit river and gets caught in the relentless tide of history.