One quibble here:
As Macgregor notes, Russia was always going to strategically win a war in eastern Ukraine for the same reason the United States would win a war in Mexico. The scale of a determined military capability eventually wins, even with a strong resistance, specifically because: (a) Ukraine is connected to a common border with Russia, and (b) the citizens in the eastern part of the country are aligned with Russia.
True enough, but the citizens in the northern part of Mexico are not aligned with the United States, and certainly don't seek to be incorporated into it. If anything, among the Hispanics in the Southwest, it would be the other way around --- the "Aztlan" concept.
If I were Mexican, I'd probably want it back too.