The US "wins" by using the Ukraine as a proxy against the "aggressive" Russians (which gains support for sanctions, etc). As such, its important for the US to not directly respond (the same logic is true with China-Taiwan). There may be two options:

1) Negotiate a truce in which Russia keeps Donbas and Crimea, Putin stands down & hopes to stay in power, repair International standing, nuclear terrorism averted.

2) Keep going with Ukrainian offensive, if Putin uses tactical nukes, give Ukrainians tactical nukes with which to retaliate.

Not sure if #1 involves Ukrainian promise to not join NATO and #2 doesn't, but I suppose with Ukraine winning (and Putin about to be deposed?) the only thing #1 has going for it is no-nukes, which just gives Putin a great toolkit going forward (remember too there are many Russian Nationalists to the right of Putin).

With China-Taiwan, more complex. With N Korea ready to invade S Korea, China threatening Taiwan, we may be forced to get more directly involved?