Let's assume a typical wank US victory in the Mexican American War; that is that the US annexes all the North Mexican Territories as well as the states of Sonora, Chihuahua, Coahuila, Nuevo Leon, and Tamaulipas. Mexico was very anti-slavery at the time and had abolished slavery decades earlier. In a lot of Timelines dealing with such a US victory, I often see the US reintroduce slavery these areas with little or no resistance by the locals.
Realistically, how difficult would it be for the United States to introduce Slavery into an area where the people didn't want it?