 This was compounded by a relentless carpet bombing campaign against major cities and a nuclear weapons strike on Monterrey.Large groups of American, British, and Japanese mechanized infantry and armored divisions were then sent in to mop up all remaining Mexican forces, which led to extended bouts of tank-versus-tank fighting and constant guerrilla warfare which lasted for months.While the American Empire was initially successful in its campaign and occupied large parts of northern Mexico, they were unable to move further into the country and suffered massive casualties at the hands of both the Mexican military and mercenary forces hired by the Mexican government.