GIVEN PRESIDENT Obama’s preoccupation with ending what he calls “a decade of war,” it’s hard to believe that the United States could be dragged into a military conflict in the western Pacific over a group of tiny, uninhabited islands claimed by both Japan and China. Probably, it won’t be. Yet thanks to a disturbing confluence of events in those countries and Mr. Obama’s own commitments, the chance that it will happen is rising. The Senkaku Islands, called the Diaoyu by China, have been under Japanese administration since 1895; for decades, China agreed to leave its claim to them on a back burner. But Japan’s nationalizationin September of three of the islets — undertaken in an attempt to head off an attempt by a nationalist politician to gain hold of them — provided China’s military and Communist leadership with a pretext for rabble-rousing.