We cannot let nuclear powers annex territory and hide behind their nukes to get away with it, because the results would be catastrophic for everyone. Or what, do we just let South Korea get annexed by North Korea tomorrow if the latter waves its nukes around?
Like, we didn't struggle against Hitler or the Communists because of the Holocaust or gulags. We fought because if Eurasia unites under various alliances, even the US can't do anything to stand up to such a power. Just as Britain fought again and again to prevent a united Europe, the US cannot allow a united Eurasia. That means standing against revanchist powers in Eurasia, of which Russia is the most immediate threat. It's that simple.
There are reasons we don't want to go all out, especially because there is the fact that Russia is not the biggest threat and we need to make sure we don't exhaust ourselves and leave Taiwan open. But we haven't gone even close to all out.