re: How is this different than a state leaving the US.

Here's some comparisons. Texas was once a "sovereign nation" like Ukraine. In quotes because its legitimacy as a truly sovereign nation is a matter of perspective like Ukraine. Texas had many Americans and pro America factions like Ukraine had pro Russia elements. The eventual annexation of Texas by America was met with condemnation from international powers and sparked a war. America won so might makes right. Of course the really big difference is the lack of connection between America and Texas the previous 1000 years like "the Ukraine" literally "the Borderland" of the Russian Empire long predating 1990 or 1917