You are telling that USA will abandon Ukraine because they abandoned South Vietnam and Afghanistan. Whereas China who betrayed USSR in the past is never going to betray anyone.
There are logical explanations for Afghanistan and Vietnam. USA has no interest in Afghanistan. 9/11 forced the issue. Vietnam is something USA get caught up in their anti-communist zeal of the 1950s and 1960s.
I am a Russophile. For both USA and Europe Ukraine is the easiest way to deal with Putin. Putin has been a headache since 2008. Every Russian tank destroyed, every Russian soldier dead is one less they have to account for. Europe is the richest continent on the planet. Europeans do not like Russian military action in Ukraine. How hard is it to understand ?
USA knows the effectiveness of its weapons. There are plenty of other places where it got tried out. The real draw is the performance and effectiveness of Russian weapons. You are right everyone is watching