You’re not getting the analogy. The US is in a position similar to the Ukraine. The southwest is similar to the areas of the Ukraine where the ethnic Russians are. Russians were shipped there to dilute the Ukrainian population, and that seems to be happening in the southwest with Mexicans. Mexicans call this area “Aztlan” and claim it was wrongly taken from Mexico. Mexico moving in to take it back, on a claim that it was protecting Mexican citizens, would be no different than Russia moving into the Ukraine to protect Russians (who Russia sent there). Keep going down the path we are going, and that area is going to split away at some point, and may even become a protectorate of Mexico.
Your analogy doesn't fit since we don't have a section of the US which is officially designated as an "autonomous region" like Crimea is. Plus, Crimea has been essentially Russian since the 18'th century. Furthermore, we don't have a region full of Mexicans which is also host to several Mexican military bases and sea ports.