This is a strange development—China's establishment over the weekend of an ADIZ, or Air Defense Identification Zone, in an expanded area of the East China Sea, eliciting alarmed reactions from Japan, the United States (which today sent two B-52s through the zone), South Korea, and other countries in the region. A few points to bear in mind as you follow the story:
1) What is an ADIZ, anyway? Many news stories have presented the ADIZ as if it were comparable to a no-fly zone, or an extension of territorial sovereignty. It's not quite that. All four words in its full title are important, including the least obvious third one: Air Defense Identification Zone. The idea is to create an area where the relevant authorities have a right to know who is flying, and where they are going. It doesn't necessarily mean that flights are going to be challenged or interfered with.
For reference, here is the way the ADIZs around the continental United States look:
In practice the U.S. zones mean that aircraft entering ADIZ space—most of the time, those bound for U.S. cities from other countries—must have filed flight plans and been cleared along their routes by Air Traffic Control. Virtually all of the world's airline flights operate on filed-and-cleared flight plans anyway, so the ADIZ makes no practical difference in airline operations. (The Chinese have said the same will apply in their new zone.) You can get extremely detailed info from the pertinent FAA regulations. They include this definition:
Air defense identification zone [ADIZ] means an area of airspace over land or water in which the ready identification, location, and control of civil aircraft is required in the interest of national security.
"Control" in this sense means subject to the directions of an Air Traffic Controller—"turn right, heading 270"—rather than something more forceful.
So this move is aggressive and expansionist, in asserting a Chinese government right to know who is traveling in its (enlarged) vicinity. But some stories have suggested that it would lead to an immediate struggle or challenge over the right to fly, which it (probably) will not.
2) Why are the Chinese doing this? As a general proposition, this is of course one more sign of worsening relations between China and Japan, focused in this case on the tiny islands both countries claim to control. As for the immediate reasons for this move, no one outside the central leadership can say with any certainty, and perhaps not even anyone there.
The lines of authority and communication between civilian and military officials in China are murky in the best of circumstances. (Remember, the People's Liberation Army technically is commanded by the Chinese Communist Party, not the Chinese state.) The concept of a civilian commander-in-chief is not built into China's governing structure. Most people think that newish president Xi Jinping enjoys more support from the military than his predecessor—which most outsiders consider to be a good thing, since it reduces the chance of the military setting policy or creating new realities on its own.*
The ADIZ move is is a big enough step that Xi Jinping himself would presumably have been aware of it, and again-presumably would have thought it a worthwhile demonstration of Chinese "strength" and refusal to be pushed around. But for now that is guesswork rather than knowledge.
3) Is this likely to do China any good? The puzzling nature of Chinese foreign policy, especially its generally self-defeating "soft power" aspects, is a subject too vast for our purposes right now. In brief: the very steps that, from an internal Chinese-government perspective, are intended to make it seem confident, powerful, and attractive often have exactly the opposite effect on audiences outside China.