The Penghu or Pescadores Islands are an archipelago of 90 islands and islets in the Taiwan Strait, located approximately 50 km (31 mi) west from the main island of Taiwan, covering an area of 141 square kilometers (54 sq mi).
The Pescadores, lying midway between mainland China and Taiwan, were the key to a successful occupation of Taiwan. In a swift campaign in the last week of March the Japanese captured the islands, preventing further Chinese reinforcements from being sent across the Taiwan Strait to Taiwan.
An excerpt from, "Penghu critical to defense of the nation: academic" Taipei Times, March 6, 2020:
The offshore Penghu archipelago is more strategically important than the former frontline islands of Kinmen and Matsu, a Taiwanese academic has said, while calling on the government to bolster Penghu’s defenses to prevent a Chinese invasion, as losing the islands would imperil the whole nation.
In a Jan. 21 article titled “The Strategic Role of Penghu in the Defense of Taiwan” in the Institute for National Defense and Security Research’s Defense Situation Monthly, Paul Huang (黃恩浩), an assistant research fellow at the government-funded institute, called on the nation’s armed forces to beef up Penghu’s defenses, which play a crucial role in national defense.
Penghu, or the Pescadores, are an archipelago of 90 islands in the Taiwan Strait that have long been a strategic location since the 17th century, with the Dutch, French and Japanese all launching campaigns against the islands, Huang said.
From a global perspective, Penghu is the first line of defense in the first island chain, which refers to the major archipelagos on the East Asian continental coast: Taiwan, the Kuril Islands, the archipelago of Japan, the Ryukyu Islands, the northern Philippines and Borneo.
Their strategic location means that China’s People’s Liberation Army (PLA) would have to take the islands in any invasion of Taiwan.
In the earliest period Taiwan was a base from which pirates harassed shipping along the China coast and in the sea lanes between Japan and Southeast Asia. Europeans became interested in Taiwan out of motives little different from those of their pirate predecessors. In the early 1620s, the Dutch seized the Pescadores [Penghu Qundao] (islands lying between Formosa and the mainland) and used them as a base from which to raid the China coast and to disrupt commerce between China, Japan, and the Philippines.Their interference with regular trade had become so serious by 1624 that the Chinese were compelled to offer a treaty that gave the Dutch commercial privileges with China.[6] In return for this the Dutch agreed to leave the Pescadores [Penghu] for a trading post on Taiwan. They were attracted to Taiwan “not only by its strategic location but also by the commercial opportunities they foresaw.”[7]Nevertheless, they “valued the island chiefly on account of its strategical position. From Formosa the Spanish commerce between Manila and China, and the Portuguese commerce between Macao and Japan could by constant attacks be made so precarious that much of it would be thrown into the hands of the Dutch, while the latter’s dealings with China and Japan would be subject to no interruptions.”[8] Other colonial and commercial powers were not long in reacting.
Further, from China’s perspective, Taiwan is one of the critical links in the so-called “first island chain” that includes Japan, the Ryukyus, the Philippines, Malaysia, Indonesia, and Australia. Beijing sees the navigational “choke points” between those islands as constraining the People’s Liberation Army’s naval access to the “second island chain” (Guam, the Marianas, the Palau island group and other small islands in the central Pacific) and from there into the open ocean far from China’s shores. China’s coastline in the East China Sea lacks the deep-water ports needed to service its naval bases located there. Its submarines must operate on the surface until they are able to submerge and dive deep when they reach the area of the Ryukus archipelagoes. If China controlled Taiwan, its submarines would have a far easier exit from Taiwan’s deep-water ports into the Pacific. They could present a new danger for Japan – which is totally dependent on the East Asia sea-lanes for its energy and other raw materials. Chinese submarines and an enhanced ability to project power into the Pacific could also present an increased threat to the U.S. Seventh Fleet, Guam, Hawaii, and even the West Coast of the United States. Moreover, to the extent China’s far-ranging navy would distract Washington and Tokyo and embolden North Korea’s already-reckless leader, it could directly endanger the security of South Korea.From a purely naval and military perspective, control of the island of Taiwan would constitute a huge strategic asset for China and a threat to the region in both Southeast and Northeast Asia as well as to the United States. Chinese control of Taiwan, its technologically advanced economy, and control of the entrance to the South China Sea it would provide would have major economic, diplomatic, and political implications for the region. There would likely be a cascading effect as regional governments recalculate their self-interests in the face of an even more powerfully situated China. Singapore might well be intimidated into a more pro-China position, consolidating Beijing’s control of the South China Sea with Taiwan in the north and Singapore in the south. Denying China that asset and that leverage is clearly in the strategic security and economic interests of the countries of Southeast Asia, Japan, and the United States.. . .
President Dwight D. Eisenhower described the reason for the Taiwan defense treaty as follows:In unfriendly hands, Formosa and the Pescadores would seriously dislocate the existing, even if unstable, balance of moral, economic, and military forces upon which the peace of the Pacific depends. It would create a breach in the island chain of the Western Pacific that constitutes for the United States and other free nations, the geographical backbone of their security structure in that ocean.In addition, this breach would interrupt north-south communications between other important elements of that barrier, and damage the economic life of countries friendly to us.