AP Photo

The Senkaku Islands dispute is the first Japan-China security crisis in seven decades of peace. This puzzling contretemps between Asia’s two giants unnerves the region, whose waters host half of global trade, and President Barack Obama faces a test. American power anchors the China-Japan balance in a tripod that is the unsung secret of East Asia’s peace and progress. Prime Minister Shinzo Abe is worried. “As a result of the trust and confidence between Japan and the United States having gone through a pretty rocky period,” he told the Washington Post on February 16, “Japan’s foreign policy clout has been declining. And the stability in Japan’s adjacent waters and in the Asia-Pacific region is being affected, with acts of provocation seen...