You can see Japan's quandary if you follow the islands down to Okinawa and beyond:
Asia Pacific Journal Japan Focus (more after the jump)
From Australia's Quadrant Magazine, April 2021 edition:
China and the Looming Warfare over Taiwan
It is hard to believe that the Japanese High Command is not saying to the Chinese High Command, secretly or otherwise, that if China attacks Taiwan, Japan will have to respond whatever the Americans or anybody else chooses to do.
Taiwan is absolutely essential to the defence and security of Japan. It is the key link in what has been called the First Island Chain running more or less parallel to the Asian mainland from Sakhalin in the north down through the pencil-like line of Japanese islands through the Ryukyu Archipelago to Taiwan through Batan and Babuyan to the Philippines and then down to Indonesia. This chain of islands prevents the Chinese from getting out of the Sea of Japan, the Yellow Sea, the East China Sea and the South China Sea into the North Pacific Ocean in any militarily significant numbers.
If China occupied Taiwan it would be able to amass significant and seriously threatening naval power both east and west of Japan. Being a long narrow line of islands, Japan lacks serious geographical strategic depth, and while it is reputedly the world’s third-largest economy, it must import all of its oil and most of its food, minerals and other essential goods. It would be very difficult, if not impossible, for Japan to mount a successful defence without massive support from the US and other allies.
These strategic realities have been occupying the minds of Japan’s best strategists for at least a hundred years, and of course they have been working diligently to maintain the chain. I first became aware of these matters when I visited Japan in the late 1960s and early 1970s and interviewed some of Japan’s major naval and other strategists.
One of the most important of these was Commander Hideo Sekino, who educated me thoroughly on Japanese naval strategic thought. He spoke not only of the First Island Chain and the strategy and tactics required to defend Japan itself, but also of the need for Japan to defend the Sea Lanes of Communications in conjunction with the US. I met him a number of times, hoping he would join the Pacific Institute of which I was Executive Secretary. The institute was warning at the time of the imminent rise of an aggressive and expansionist China and the need for Australia and its allies, especially the US and Japan, to protect the strategic waterways running through Indonesia and the South China Sea (see the 1970 Pacific Institute Resolution and its regional military co-operation section which I wrote with Australian Brigadier Ted Serong, the most insightful “general” of the Vietnam War).
Today, having witnessed over the last ten years China’s aggression in the South China Sea and having heard Xi Jinping’s stated ambitions of “world domination” within the next few decades, Japan, already with the world’s fourth-strongest military forces, has embarked on a massive military expansion—and being the world’s third-largest economy, it can afford to do so.
On October 20, 2020, the Japan Maritime Self-Defence Force (that is, Navy) launched its twenty-second submarine in a little over two years, this one the first of the new “Taigei” class. As of recently, the Japanese Navy, the third-largest navy in the world, also operated two multi-purpose operation destroyers (de facto aircraft carriers), four helicopter carriers (called helicopter destroyers), twenty-six destroyers, ten small destroyers (or frigates), six destroyers escorts (or corvettes), and thirty mine counter-measure ships among many other vessels.
Japan’s very impressive ships are equipped with the best weaponry the US can supply including the Aegis missile system. Japan also maintains a large naval air force, including over 200 aeroplanes and 145 helicopters. Most of these aircraft are used in anti-submarine warfare operations. The Air Force has the best fighter bomber and other aircraft and weaponry the US can supply. The Army is equally well equipped. And we know how well the Japanese can fight. One of Japan’s biggest problems is an ageing and declining population, making military recruitment difficult. But it has also become more open to more US forces being based in Japan within striking distance of Taiwan and to more visits from the nuclear-armed US Pacific fleet....
....MUCH MORE
And naturally enough there is a complicating factor in the competing claims to the Senkaku islands:
For some backstory here is the source of the two maps, Asia Pacific Journal's:
Barren Senkaku Nationalism and China-Japan Conflict
