Because annexation is made simpler if you offer incentives to those that could cause you problems. It's the same reason why higher ranking royalists were given lands in England. If you're part of the system, you have a vested interest in maintaining a status quo.

As it pertains to citizenship for Africa Americans there was little risk or danger of failing to do so causing rebellion or uprising from those individuals whereas Mexicans living in Texas could cause many headaches inside the recently annexed territory.

So it's another example of incentivizing systematic integration. If you have benefits for taking or not taking certain action, you're less likely to cause trouble.

It has nothing to do with the comparative morality of civil rights.

The Treaty of Guadalupe-Hidalgo ended the Mexican-American War in 1848. The terms of that agreement granted citizenship to approximately 80,000 Mexicans who lived in Texas. Individuals had the option to opt-out of citizenship. The treaty also provided for the establishment of the New Mexico Territory.^1 It is worth noting that African Americans and other groups did not have full citizenship or rights granted by the Constitution. The treaty only guaranteed citizenship for those Mexican citizens how elected to stay in Texas. The Dred Scott decision was handed down in 1857.

The Wilmot Proviso was a failed attempt by Pennsylvania Congressman David Wilmot to outlaw slavery in all territories obtained from the Mexican-American War.

1. Linda C. Noel, "'I am an American': Anglos, Mexicans, Nativos, and the National Debate over Arizona and New Mexico Statehood," Pacific Historical Review.

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