Not sure what you are implying by "fought for their lives", as this seems to indicate that you think China would somehow attack the US and/or Japan and therefore somehow the US and Japan would be legitimized and justified in fighting China. Nothing could be further from reality here, and here I detect also a distinct whiff of the age-old Yellow Peril through which Westerners typically view China and its rise. There is no realistic scenario out there in which China initiates military hostilities against either the US or Japan, whether over Diaoyutai or SCS or whatever other scenario. Let's be honest here: the only real threat is from the US against China, such as may happen if China militarily reunites Taiwan and the mainland (which I personally think will probably occur at some point in the not too far distant future). It is important to note that in this case, neither Japan nor the US would be attacked, and certainly neither would be "fighting for their lives". This would be a military conflict of the US's own making, and initiated by the US, and followed along solely by its lackey Japan. SK would almost certainly sit this one out, and Australia would have the perfect cover to not get involved.Sometimes you do not get to choose...sometomes your are forced to play.
THiose who believe that South Korea would stand by while the US and JApan fought for their lives are, IMHO, sorely underestimating the SKOR mind set.
They are and have been solid allies for decades...and I believe if it came down to it, they would stanf by that alliance.
Like Kwaig, I pray we never have to find out...but as I say, and as we have found in the past...somtimes you are not left with a choice.
As it is, US, JMSDF, ROKN, and the RAN together are a very powerfu...and for the forseeable future, an allied force that is not about to be overpowered at this stage.
If you willingly throw yourself and your friend into the lion's den when the lions are fighting, who is going to pity you when you get mauled?