The Japanese government is outraged after Russian President Vladimir Putin visited a tiny island north of Hokkaido, a territory that Tokyo has insisted was illegitimately stolen from the country in the final days of World War II.

Putin toured the island, known in Russia as Iturup and in Japan as Etorofu, on Thursday. It is part of an archipelago with three other islands known in Japanese as the “Northern Territories” and in Russian as the “Kuril Islands.” It was a politically charged visit, as Putin asserted just the day before that Russian ownership of the region is “reality” that is “enshrined in international documents.”