For the last four months, China has been increasingly aggressive towards Japanese reconnaissance aircraft (mainly P-3s and EP-3s) flying in international waters. For example, before last September, Chinese warplanes generally stayed out of Japans ADIZ (Air Defense Identification Zone), which covers a lot of international waters, and in many places extends to the mid-point between Japan and China. Any aircraft that enter the ADIZ and refuse to identify themselves, have to be checked out by fighters. This happened, because of Chinese intrusions, 44 times last year.
All this is apparently because of growing Chinese aggressiveness around the disputed Senkaku islands. Things began to heat up last September, when a Chinese fishing trawler captain was arrested for threatening (with ramming) two Japanese patrol boats near a disputed bit of ocean (that apparently contains large quantities of natural gas and oil under the seabed). Japan later freed the Chinese trawler captain, but refused to apologize for the arrest. China has increasingly used commercial ships to "intimidate" foreign warships entering waters that the Chinese consider their own (even though international law does not agree with them.) Japan has stood up to the Chinese pressure, and is now planning to put small garrisons on some of the disputed islands.
This particular dispute goes back a long way. China and Japan have been squabbling over ownership of the uninhabited Senkaku Islands in the East China Sea with increasing fervor. Six years ago, Japanese officials took control of a privately owned lighthouse built on one of the eight small islands, and warned China to stay away. The Senkakus are actually islets, which are 167 kilometers northeast of Taiwan and 426 kilometers southwest of Japan's Okinawa and have a total area of 6.3 square kilometers. Taiwan also claims the Senkakus, which were discovered by Chinese fishermen in the 16th century, and taken over by Japan in 1879. They are valuable now because of the 380 kilometer economic zone nations can claim in their coastal waters. This includes fishing and possible underwater oil and gas fields. A conservative Japanese political group built the lighthouse in 1986, to further claims of Japanese ownership. Currently, the Japanese have the most powerful naval forces in the region, and are backed up by a mutual defense treaty with the United States. China was long dissuaded by that, but no more.