"If the world has changed enough, the West (as it did in 1939) will stand up for itself "
That's a view often put forward here but of course its based on a historical inaccuracy. The "West" didn't step up against Hitler in 1939 - only the Poles, the British & French and their respective Empires did and they were only fighting the Germans (and Austrians). Most other European countries, but not all (no Ireland, Switz, Sweden, Spain or Portugal), were eventually dragged in from 1940-41. The US didn't come in for over 2 years and various other countries came in even later - the Russians finally joining the war against Japan a week or so before the end. If Hitler hadn't kept invading everyone in sight it would have remained a restricted war, probably with restricted results. Same with the Japanese - if they'd stayed in China or even only invaded the Russian Far East would the USA have come in?
Western countries only "stood up" when there was literally no alternative or when they were actually invaded. I suspect the same will happen with China