(UPDATE-2) Two B52 bombers out of the base in Guam flew over the airspace over Senkaku Island as part of regular training, without informing the Chinese. (from Wall Street Journal, 11/26/2013)
(UPDATE) According to Yomiuri (11/26/2013), China's newly declared ADIZ includes the airspace used by the US military stationed in Japan. The US Department of Defense spokesman said that the US military has been conducting the training in the area, and the US will not submit flight plans to China as demanded by China.
China seems to think they have Japan (and the US behind it) all figured out, and is confident that it can get away with it.
The Air Defense Identification Zone, or ADIZ is already in place by the Japanese government over Senkaku Islands. Japan's ADIZ is almost identical to the airspace set by the US military, according to the Japanese wiki on ADIZ.
From The Guardian (11/23/2013; part, emphasis is mine):
China imposes airspace restrictions over Japan-controlled Senkaku islands: China says it is exercising self-defence right but experts say move will exacerbate tension between the two nations
China has tried to establish its authority over Japanese-controlled islands in the East China sea by demanding that all aircraft flying in the region obey its rules or face "emergency defensive measures".
The East China sea air defense identification zone came into effect from 10am local time on Saturday when the Chinese defence ministry issued a map of the area, which includes the uninhabited East China sea islands.
The ministry said all aircraft entering the zone must notify Chinese authorities and warned they would be subject to emergency military measures if they did not identify themselves or obey orders.
It said it would "identify, monitor, control and react to" any air threats or unidentified flying objects coming from the sea.
"China's armed forces will adopt defensive emergency measures to respond to aircraft that do not cooperate in the identification or refuse to follow the instructions," it added.
...But Narushige Michishita, director of the security and international studies programme at the National Graduate Institute for Policy Studies in Tokyo, said: "China is playing a dangerous game here. It is certainly an escalatory action and might prolong and exacerbate the ongoing tension."
(Full article at the link)
Japan's Asahi (11/24/2013) has actual quotes from both Japanese and Chinese officials:
...“Japan’s ADIZ [Air Defense Identification Zone] is a ‘trench in the air’ that is illegally occupying our country’s territory. To break the ADIZ is the first step to protect the sovereignty of the Diaoyu Islands (Chinese name of Senkaku Islands),” Liang Fang, a professor of strategy at PLA (People’s Liberation Army) National Defense University, told the Global Times this month. The newspaper is published under the auspices of the People’s Daily, mouthpiece of the Chinese Communist Party.
...China is also bolstering its military capabilities, especially by building aircraft carriers and developing new ballistic missiles, partly to deter U.S. forces from approaching areas around Senkaku Islands.
“China’s establishment of the ADIZ seems to be a way of discouraging U.S. military aircraft from approaching Chinese territorial airspace,” a Japanese government official said.
On the day that China announced its ADIZ, Japanese Defense Minister Itsunori Onodera held an executives’ meeting in the Defense Ministry.
Those who attended the Nov. 23 meeting confirmed that the ASDF will continue to patrol the skies around Senkaku Islands in the same way as before. Those activities include scrambles if Chinese aircraft enter Japan’s ADIZ.
After the meeting, Onodera voiced extreme concern about China’s latest move.
“The unilateral declaration of the ADIZ without having any consultation with neighboring countries is an issue that we have to protest against. If an incident takes place in the ADIZ, it could trigger a dangerous situation,” Onodera told reporters.
“I wonder whether Chinese aircraft will scramble when a (Japanese) SDF plane enters airspace claimed by China as its ADIZ. We need to see how China will operate its ADIZ,” a Japanese Defense Ministry official said.
(Full article at the link)
After the last-minute deal on Syria where the US president was outsmarted by the Russian president, and the latest nuclear deal with Iran despite the protest from the US's allies in the Middle East, China may be thinking the US is such a pushover and a wimp, and now's the time.
South Korea is not too happy with China's newly expanded ADIZ either, which now covers the territory claimed by both South Korea and China, according to Asahi (Japanese).