The island Dokdo has never been the land of Japan. There are clear evidence in history. During the Chosun dynasty, there was order 41, a order to accredit that Ullengdo and Dokdo was land of Chosen.
Japan is arguing that Dokdo is Japan's territory since World War 2, but it was only a part of colonizing Korea and it doesn't have any evidence due to international law.
Also, the Korean government is the one that is protecting Dokod; this means that Dokdo is not a territory of Japan.