On Friday, a group of six Chinese marine-surveillance ships entered territorial waters around the remote Senkaku Islands, which are claimed and administered by Japan as sovereign territory. It was the latest and most serious escalation in the dispute over the small but potentially valuable islands; they are also claimed by China, where they are known as Diaoyu. …

Officially, the U.S. takes no position on the Senkaku-Diaoyu dispute or the many other conflicting territorial claims that are upsetting the region. Under the U.S.-Japan Security Treaty, the U.S. is obligated to respond to any attack on Japan or its territory. Pressed to declare whether that security umbrella includes Senkaku/Diaoyu, U.S. officials stated publicly that the treaty applies to “all areas under Japanese administration” — a seemingly clear nod to Senkaku/Diaoyu.

But Sato says that’s not clear enough. The alliance also calls for Japan to take “primary responsibility” for territorial defense. That could give the U.S. a loophole to avoid confronting its most important trading partner and leave Japan on its own, he says.