Because West gave security assurances to Ukraine back in 1994.


Budapest memorandum on security assurances to Ukraine, Belarus and Kazakhstan

If you say in a written statement you will defend someone against threats by nuclear powers in exchange for giving up their nuclear weapons, it is prudent to do so when that country is threatned by nuclear powers. That gives you international credibility, because it means your word actually means something.

Russia also signed the agreement. Yes, the agreement also included a clause that said the signatories will never, ever threaten Ukraine, Belarus or Kazakhstan. That pretty much eliminates the possibility of Russia being seen by anyone as a credible, honest partner until at least the end of this century.

Allowing Russia to take Ukraine unchallenged will damage the credibility of the west, China will also attempt to take Taiwan

So that Russia can ruin Ukraine completely abi?
Why do you have allies if they can't help you in time of trouble

God1000:
Allowing Russia to take Ukraine unchallenged will damage the credibility of the west, China will also attempt to take Taiwan

Exactly!
So on point. What Russia has started could easily spark the 3rd world War.