Do you have a source regarding the theory that the race was the primary factor in deciding the territory following the Mexican American war? I haven't encountered this example before. I'm obviously skeptical but willing to read more.

As a counterpoint, the last several centuries are full of examples of countries establishing territories or colonies in foreign lands with different ethnicities. In those cases, the local population was generally not given much power for self governance, if at all.

The US has a pretty long history of returning militarily conquered territory back to locals for self governance. These countries usually remain within the US sphere of influence of course, and often retain a US military presence, but we dont call it occupation or the territories colonies. Instead we call them allies (e.g. Germany, Japan, the Philippines, S Korea, ect).