Russia Ukraine war, page-228406

IMO the USA would use tactical Nukes/conventional Nukes
if the US home territory was under attack and that conventional
defence systems were failing/likely to fail.

IMO the same applies to Russia or even any Nuke armed country
with lesser Nuke abilities such as Israel, North Korea , Pakistan
etc etc.

And of course, the rationale of AUKUS is that not alone are the subs
Nuke propelled, they will have the ability to carry either US or UK Nukes
capable of complementing US first or second strike capabilities on either
China, Russia or China-Russia combined. (Our Claytons' Nuke Option)