One major factor people are not considering is that after the annexation Mexican's were given the choice to stay or leave, if they stayed they were given US citizenship but property rights would not necessarily be protected. A very large number chose to leave. This would easily have been the case elsewhere. Assimilating them would not have been as difficult as people think, given the silver rush they would have been flooded with American's. The United States was able to capture and maintain control of Mexico city, a urban area populated with millions, I do not think it would have been too hard for slave owners to protect their newly expanded institutions.
As for Yucatan, slavery would not have expanded there, not initially at least. It probably would have been a bit difficult to incorporate as state & would have stayed as a protectorate for while, however, a large minority out of its 500+ population at the time where Urban Whites who had enclosed themselves in cities as they feared being driven out by the Natives. If US intervened the British would not be supplying arms to the natives to avoid annoying the US, this of coarse is as long as the US does not expand slavery there.
As for Rio-Grande, its revolution was too heavily influenced by the pro-slavery Texas, so I suppose after being incorporated by the US it would have gone down as a slave state.
As for Yucatan, slavery would not have expanded there, not initially at least. It probably would have been a bit difficult to incorporate as state & would have stayed as a protectorate for while, however, a large minority out of its 500+ population at the time where Urban Whites who had enclosed themselves in cities as they feared being driven out by the Natives. If US intervened the British would not be supplying arms to the natives to avoid annoying the US, this of coarse is as long as the US does not expand slavery there.
As for Rio-Grande, its revolution was too heavily influenced by the pro-slavery Texas, so I suppose after being incorporated by the US it would have gone down as a slave state.