1) This is the key point that John Mearsheimer just recently made. I paraphrase: It doesn't matter if we think Ukraine entering NATO is an existential threat to Russia. It matters that Russia believes it.

2) Obama understood that Russia will ALWAYS have escalatory dominance in Ukraine.

3) Russia has the largest nuclear arsenal on the planet. After a year of war, you'd have to be a fool to believe that Russia cannot deliver nuclear warheads via missiles and rockets.

4) Russia is the only country right now with proven hypersonic rocket technology. Also right now there is no technology to defend against them, despite claims in May by Ukraine to have downed a Kinzhal with a Patriot battery.

Take 1-4 together and it makes it clear that Obama was correct to not escalate any more in Ukraine than we did by overthrowing their government in 2014. We cannot win in Ukraine against Russia.

This does not mean Russia is all powerful. It does not mean Russia is good. It just means we can't win in Ukraine against Russia, because Russia will not stop.

It is similar to if (when?) the US invades Mexico. Russia would never be able to stop us. Or China. We'd mass troops in El Paso, Laredo, and Brownsville, with plenty of reserves held hundreds of miles from the border. We will always have escalatory dominance.

It is also similar to what is going to happen if the US tries to coup Taiwan. China will just blockade the island. Yes, we have a lot of bases in the area. But China is just across the strait with their entire country. They will always have escalatory dominance there.