Nothing new really as they do the same thing with their water space as well but the U.S Navy has been sailing into it for years.
Correct. It is an old tension that surfaces from time to time. Japan controls those islands, and they aren't going to give them up. The fishing is very good and the shipping lanes are important to them. Those islands are actually NORTHwest of Okinawa and are in Okinawa Prefecture.
PM Abe is getting tougher, which helps his reputation in Japan. And Li Keqiang gets to appear tough to his own countrymen. The funny entity is Taiwan, as they also "claim" the islands
Last week, Japan scrambled fighter jets three times after Chinese military aircraft flew near Japanese airspace. The two countries have argued for decades over the islands. The latest round of saber-rattling started in 2012 when the Japanese government bought three of the islands from their private Japanese owner. That made China angry. "You bought something you cannot own".
I watch a lot of news on NHK in Japanese; this is a recurring frequent news item. The history between those countries has been troubled for a LOOONG time. Manchuria. Ouch. One of my old neighbors in Tokyo was a soldier in Manchuria. He'd open up and tell me some wild stories late at night, after a lot of sake.