Sino-Japanese tensions are boiling over again. The proximate cause is five uninhabited islands and three rocks, called the Senkaku by Japan, which administers them, and the Diaoyu by China, which claims them. The real cause is national pride and unfinished business left over from the war.

The latest flare-up follows the visit to the islands by Japanese nationalists after police arrested and deported 14 Chinese activists who travelled by boat from Hong Kong to plant flags last week. The Japanese display triggered angry demonstrations in at least half a dozen Chinese cities. In Chengdu, where thousands marched, one banner, reflecting enduring anger about Japan’s wartime aggression, read: “Even if China is covered with graves, we must kill all Japanese.” One Chinese general suggested Beijing should send 100 ships to defend the islands.

In Japan, Shintaro Ishihara, the nationalist governor of Tokyo, has been collecting donations to buy the islands from the family that owns them. If he succeeds, or if the Japanese government heads him off by purchasing them instead, the dispute may escalate. This is no local scrap. Tokyo says Washington has confirmed that the Senkakus are covered by the US-Japan security treaty. In theory, that would mean American soldiers risking their lives to defend the uninhabited islands if China were foolish enough to attack.