The United States could bounce back from the fall of Taiwan to Communist rule. It would have far more dire consequences for Japan. Consider this post a short, informal primer on why this is so.
Ian Easton explains the PLA’s view:
The Course Book on the Taiwan Strait’s Military Geography is a restricted-access PLA manual, used to teach senior officer seminars in Beijing… This source [informs] readers that Taiwan is a chokepoint of great utility for blockading Japan. The Taiwan Strait, it notes, is a Japanese maritime lifeline that runs from Europe and the Middle East, and based on PLA studies, Japan receives 90 percent of its oil imports, 99 percent of its mineral resources, and 100 percent of its nuclear fuel needs from ships that travel across these sea lanes. In total, 500 million tons of Japanese imports pass by Taiwanese waters each year, with 80 percent of all Japan’s container ships traveling right through the Strait, the equivalent of one Japanese cargo ship every ten minutes. Consequently, these waters will, "directly affect Japan’s life or death, its survival or demise."
PLA intentions and plans for a conquered Taiwan are made plain in another internal document, The Japanese Air Self Defense Force, a handbook studied by mid-career officers at the PLA Air Force Command College in Beijing. The stated purpose of the text is to help Chinese pilots and staff officers understand the strengths and weaknesses of their Japanese adversaries. Buried amidst hundreds of pages of detailed maps, target coordinates, organizational charts, weapons data, and jet fighter images are the following lines:
As soon as Taiwan is reunified with Mainland China, Japan’s maritime lines of communication will fall completely within the striking ranges of China’s fighters and bombers…Our analysis shows that, by using blockades, if we can reduce Japan’s raw imports by 15-20%, it will be a heavy blow to Japan’s economy. After imports have been reduced by 30%, Japan’s economic activity and war-making potential will be basically destroyed. After imports have been reduced by 50%, even if they use rationing to limit consumption, Japan’s national economy and war-making potential will collapse entirely…blockades can cause sea shipments to decrease and can even create a famine within the Japanese islands. 
The first PLA document Easton quotes here has the statistics slightly wrong: the larger part of Japan’s energy imports travel to the south of Taiwan through the Bashi channel, in the Luzon strait. To get a sense for what those shipping lanes look like, here is a map of A.P Moeller-Maersk, Mediterranean Shipping Co. and CGM SA’s Japan bound shipping routes:
A decade ago an underseas earthquake in the strait knocked out the internet in Taiwan, Japan, South Korea, and Eastern China. I will admit that I do not know how easy it would be to isolate the cables headed towards Japan and knock them out of service, but I am interested in finding out. If you work in that industry or have expertise in underseas infrastructure, please sound off in the comments!
 Ian Easton, The China Invasion Threat: Taiwan’s Defense and American Strategy in East Asia (Washington DC: Project 2049 Institute, 2017), 27-28.
Euen Graham, Japan’s Sea Lane Security: A Matter of Life and Death? (Nissan Institute/Routledge Japanese Studies, 2006), pp.23-26.
 The Senkaku Islands are also easier to assault from Taiwan than anywhere else. However, I tend to agree with Todd Hall’s assessment that the Senkaku island dispute is more about the symbolics of honor than strict military utility. See Hall, "Why the Senkaku/Dioyu Islands Are Like a Toothpaste Tube," War on the Rocks (4 September 2019).
 E. I Chen, "Japan’s Decision to Annex Taiwan: A Study of Ito-Mutsu Diplomacy, 1894-95," The Journal of Asian Studies (1977), vol 37, iss. 1, pp. 65-67; Graham, Japan’s Sea Lane Security, passim.
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