The United States has three chains of islands in Pacific Ocean. Among them, first chain of islands starts from Japanese archipelago in north, passes through Ryukyu Islands and Taiwan, and reaches Philippine Islands and Greater Sunda Islands. The second chain of islands is centered on Guam and consists of Japan's Ogasawara Islands, Iwo Islands, and Mariana Islands of United States. A line of defense was set up with Aleutian Islands and Hawaiian Islands as core. The United States has built a large number of military bases on these island chains, almost turning Pacific Ocean into an inland sea.
The three main island chains were created due to Cold War, but they are still being reused today and have a big impact on rise of China. If China wants to break out of inland sea and into ocean, it must break out of encirclement of US island chain. Similarly, if China does not have islands in Pacific Ocean, China's waters will always only be around mainland, which is very inconvenient. Before opening of new sea route, ocean became a battlefield for all countries, but China missed it.
If you want to break first chain of islands, you need to start with Ryukyu Islands. The Ryukyu Islands lie in East China Sea, between China and Japan. In ancient times, Ryukyu was a vassal state of China, and in modern times it has been occupied by Japan. After World War II, Roosevelt attempted to transfer Ryukyu Islands to China, but Chiang Kai-shek refused, causing China and Ryukyu Islands to miss opportunity.
In 1970, United States transferred Ryukyu to Japan. Japan owns Ryukyu Islands, which is extremely unfavorable for China's maritime position. Japan is using Ryukyu as a springboard to spy on Taiwan, fight over Diaoyu Islands, and want to swallow East China Sea. If China succeeds in owning Ryukyu Islands, then East China Sea will be surrounded by China on three sides, practically becoming an inland sea, and there is absolutely no chance for Japan to intervene. At same time, Chinese Pacific Fleet can pass through Ryukyu, go straight into Pacific Ocean and go to depths of the sea.
However, owning Ryukyu Islands is not enough to have a big voice in Pacific. The Ryukyu Islands are only a fortress in East Asia, an unsinkable aircraft carrier, but they cannot move towards Pacific Ocean. At this time, we will see that Ryukyu Islands are further east, and these are Marianas. From geography, we learned that deepest place in world is Mariana Trench.
The United States has military bases in Mariana Islands, most famous of which is Guam at southern tip. This base is like a supergun planted in Far East, constantly threatening Asia-Pacific region. Therefore, to ensure security of Asia-Pacific region, this point must be pulled out.
If China owns Mariana Islands, then threat to China's maritime area will be practically eliminated, and Chinese fleet will be able to reach depths of Pacific Ocean at any time. At this time, territory of China extends from Mariana Trench in east to Himalayas in west.
Of course, if China succeeds in regaining outer northeast, then northeast region of China will need a wider sea area, so it must conquer Thousand Islands. The Kuril Islands currently belong to Sakhalin State of Russia, so they are at least nominally part of Sakhalin Island. With Outer Northeast, Sakhalin Islands and Thousand Islands, China's northern air route can be opened. Moving north from mouth of Heilongjiang, it has closest air route to United States. Thus, economic situation of Northeast China will be greatly improved.
Again, if China takes over these islands, wouldn't that mean Japan is surrounded by China? That's right, when Japan looked around, it was all of China, and she couldn't help but cry.