Sorties by Japanese warplanes, to intercept foreign aircraft coming very close, to Japanese air space, were up 29 percent last year, to 386. This is the largest number since 1991 (when there were 488 sorties). With the end of the Cold War, the number of intrusions fell in the 1990s, but in the last decade, the number has increased. The all-time peak was 1984, when there were 944 interception sorties.
These intrusions have been increasing sharply over the last three years. Early on, the Japanese launched many aircraft for each intrusion. For example, in 2008, a Russian Tu-95 entered Japanese airspace, near an uninhabited island about 600 kilometers south of Tokyo. Although the Russian aircraft was in Japanese airspace for only about three minutes, the Japanese launched 22 aircraft to intercept. This force included two AWACs aircraft and twenty fighters. It had been two years since a Russian aircraft entered Japanese airspace without permission, and that explained the massive response. But as the intrusions increased, the number of interceptors sent out declined. One explanation for all the Russian activity has been Japanese diplomats pressuring the Russians to return the Kurile islands (off northern Japan). This has caused a lot of tension, and the Russians have responded with more aerial activity. This sort of thing also goes over well inside Russia.
These four islands were seized at the end of World War II, and the Russians kept them. The Kurils had been occupied by Japanese for centuries, but when Russia reached the Pacific coast in the 17th century, they began to send ships down to the Kurils. In 1875, Japan and Russia signed a treaty settling claims in the area. Japan acknowledged Russia's claim to the larger Sakhalin island to the north, while Russia acknowledged that the Kurils belonged to Japan.
After World War II, Russia expelled the 17,000 Japanese inhabitants of the four Kuril islands Japan wants back. Russians were brought in, and about 16,000 of them (including many Ukrainians, Koreans and so on) currently inhabit the islands. There's not much economic value to the Kurils, but the Russians are still hacked off at losing a war to Japan in 1905, and to Japanese soldiers occupying parts of eastern Russia after World War I. Japan and Russia had a non-aggression treaty for most of World War II. But Russia declared war on Japan on August 8th, 1945, and promptly invaded Japanese occupied northern China (Manchuria). Japanese surrendered to the United States a month later. You could say that Japan and Russia have a lot of unresolved issues, and all that aerial activity off northern Japan is a result. If nothing else, it gives pilots on both sides lots of practice.
A similar situation is developing (with China) over the Senkaku Islands near Okinawa. China and Japan both claim these uninhabited islets, which are 167 kilometers northeast of Taiwan and 426 kilometers southeast 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.