Senkaku is an Okinawa chain of 150 islands in East China Sea contested by China and Japan. Senkaku are small but strategically significant and are guarded and controlled by Japanese government.
According to Japan, it was Japanese nationals who pioneered a settlement in these islands in late 19th century and China started to claim these islands only after 1968 when UN cited the possibility of oil reserves in the area.
However, China and Taiwan, on the basis of documentary evidence, claim that the territory was held by China before First Sino Japanese War (1894) and Japan later captured it, which should be returned back to China.
Japan believes that by controlling Okinawa chain of islands China want to dominate the 150 km wide Miyako strait, between Senkaku and Miyako islands. Miyako strait would give Chinese army channel to West Pacific, which is dominated by US Navy.
Chinese fishing ships had started entering the area around Senkaku since 2012 but their entry is checked by Japanese navy and troops guarding the Senkaku/Diaoyu frontline.