well yes, the Soviets were fighting for/in their own territory(not for another country) So they had every reason to engage in war/shed their blood for this/same with China. Its also why they didn't intervene when China invaded Vietnam years later (even though they had a mutual defence agreement with Vietnam). The thing is no major power will accept to shed its blood for you, more even so when it means fighting/putting its own territory in danger for you. The risks far outweight any gains from this, even more so when it means fighting a nuclear power as well. except that country itself is being threatened then yes. If not, then its a NO, NO.
However this isnt the case with the contested islands in SCS which isnt a U.S territory and which several countries are claiming it as theirs. So why would the U.S risks a major war which will affect its economy and maybe even territory over these small islands with no real value?lool As i said as it stands the U.S already controls Asia from its military bases in Japan, Korea, acess to bases in Philippines to diego Garcia in Indian ocean facing India, to its bases in Hawaii etc It simply has no reason to risk a major war for these islands, if not even during the scarboroh shoal incident it would have intervened(having a mutual defence treaty with Philippines). The reason is its simply too trivial a reason for the U.S to engaged itself for these small matters. However, if China or even Russia to make any move against/threaten U.S military bases in Japan/South Korea. Diego Garcia or hawaii, then thats another matter altogether, which will justifiably require a swift military response from the U.S no matter if its means going to all out war(even a nuclear one).