> For China it's about nationalism, for US it's about protecting allies/upholding treaties and protecting democracy from the strongest authoritarian regime.

I doubt this is true for China (I very much suspect economic concerns trump any other concerns for them as well), but I am quite convinced you are wrong about the USA - one of the biggest supporters of non-democratic regimes in the world. There is little in US history to suggest they have any preference for a democratic regime over a subservient autocratic one. They are also extremely clearly uncaring of international treaties.

And make no mistake: the USA is coordinating Ukraine's defense because it sees it as a good chance to weaken Russia, not out of some deep care for the people of Ukraine.