TwoTribes
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Along the coastline from Singapore to Haiphong, the sea area extending 200 kilometers offshore is mostly shallow, with the exception of the deeper waters near Quy Nhon to Nha Trang. This deeper section also falls within the coverage range of Hainan Island's naval and air forces. Merchant ships can fully transport goods along the Singapore-to-Haiphong coastline to areas near China's coast, under the protection of coastal nations.I did a quick search for South China Sea, and it appears the central basin is very deep, plenty of 100m+ deep water for SSNs to operate in.
The Sunda Shelf is a pretty big area of shallow open water that would be difficult for an SSN to operate effectively. However, that is hemmed in by chokepoints to the south and the South China Sea SSN playground to the north, so wouldn't give China much sea room to sail imports through.
Once they enter the waters north of Da Nang and Sanya, merchant ships are almost entirely safe. Would you attack any merchant ships entering or leaving this area? Drag Vietnam into the war? Moreover, the ships entering these waters won’t just be from China and Vietnam—they will also come from Southeast Asia, South Asia, and any other parties looking to benefit from the situation. Are you certain you want to sink them?
Sea transport is just one method. Southeast Asia shares a land border with mainland China, so goods don’t necessarily need to enter the South China Sea—they can be unloaded and transported overland to China. China is constructing and investing in various highways, railways, and canals across the Indochinese Peninsula.
What matters is not whether merchant ships entering the South China Sea risk being sunk by submarines, but rather that once resources enter Southeast Asia, they will reach China through various means. Similarly, when Chinese semi-finished products and components enter Southeast Asia, they will be distributed worldwide through various channels. Southeast Asia will not cooperate with your blockade.
Depth is not the only factor affecting submarine operations.
If blockading the South China Sea were easy for the U.S. Navy, it would be just as easy for the Chinese Navy. The South China Sea is not an open water; it is connected to the outer oceans by several straits. The Chinese Navy's submarines and unmanned underwater vehicles can easily patrol and lie in wait nearby, ready to target U.S. nuclear submarines entering the South China Sea.
The South China Sea is close to Chinese bases but far from American ones. China's infrastructure in the region far surpasses that of the United States. On Hainan Island, China has airfields and naval ports, has fortified several reefs into strongholds, and has established an underwater monitoring system. What about the United States? You lack sufficient air and naval bases in the South China Sea. Perhaps you could rebuild your facilities in the Philippines, but that would take time, and even if completed, the support capacity may not match China's. Without adequate infrastructure, you cannot provide sufficiently powerful surface fleet and air support for submarine operations. This means China's surface ships and anti-submarine aircraft can more comfortably hunt U.S. nuclear submarines, and the U.S. lacks the means to counter them.