This is wrong analogy. If the USSR advanced into European countries, it would be naked invasion against other sovereign countries and the invaded countries would fight them with all they had. The USSR did not have the will to launch such offensive. On the other hand, when Nazi Germany invaded the USSR, the Russian people made huge sacrifices to fight against the Nazi. I'm pretty sure you're very familiar with that part of the history.
The notion that the US's "credibility" would be on the line and would be "kicked out of Western Pacific" is ludicrous and is argued to deter China's inevitable unification of Taiwan. Weighting someone's credibility, even that of the almighty U.S.A., against Chinese territory and sovereignty is way out of proportion. That once China unifies Taiwan, the US would have to get out of Western Pacific (what does that even mean?) is weird logic. We're not talking about China invading Japan here, which is totally different from China recovering Taiwan.
So you're confusing Taiwan, a legitimate territory of China, with Europe or Japan, both of which are independent sovereign countries of their own.
Yes a better analogy would be the ongoing Armenian–Azerbaijani conflict over Nagorno-Karabakh. Russia very wisely doesn't think a refusal to defend Armenian control over Nagorno-Karabakh means the collapse of CSTO and end of Russian influence in Caucasus. It's only American hubris that tricks them into thinking a final resolution to the Chinese civil war in Beijing's favor means the end of American influence in Asia-Pacific. It's especially laughable when none of America's allies in the region wants to get militarily involved in a conflict over Taiwan.