That’s how you’d like to envision the USSR, but the fact is it was NOT just Russia.
So. Vietnam asked for assistance, and we responded. The ethnic Russians asked Putin for assistance, and he responded. They were being marginalized in areas where they were clear majorities, with the use of Russian alongside Ukrainian in government business (for example), so they decided they didn’t want to be part of Ukraine anymore.
I won’t carry water for Putin or the Russians living in the Ukraine, but look into the causes of this conflict before assuming this was just a bully attacking a neighbor. The Russians within Ukraine’s borders see a different bully.
Putin is attempting to conquer Ukraine and absorb its wealth and agriculture and important coastline into Russia as a major step toward rebuilding the Russian evil empire.
Sweden and Finland saw something that you aren’t and even Switzerland and Austria are discussing seeking NATO membership to avoid what they see as the long-term goals that Russia has of step by step rebuilding empire.