Japan grows nervous about the U.S.

Nobody has made the Japanese more nervous than Mr. Trump. He has spoken cavalierly about Japan needing to defend itself, while setting in motion a sometimes chaotic process of negotiation with North Korea that has brought the two Koreas closer together. The U.S. decision in October to cancel military exercises with South Korea has to make the Japanese doubly worried, even though Japan has its own disputes with South Korea over the sovereignty of the Liancourt Rocks islets and “comfort women” abused during World War II. If the U.S. weakens its military ties with one ally, Japan recognizes, it may do so with another. And since Mr. Trump abruptly abandoned the Trans-Pacific Partnership, Japan has been within its rights to question the future of American leadership.

Then there’s Taiwan. Because the nearby island was under Japanese occupation for 50 years (1895-1945), and was its first overseas colony, Tokyo has always taken a keen interest in the fate of Taipei. If the day comes—and Japan fears it’s approaching—when the U.S. can no longer credibly defend Taiwan against a Chinese attack, Japan will only feel more besieged and insecure.

Survey the region. Developments in China, the Korean Peninsula and Taiwan mean that Japan has no one else to turn to but the U.S. The specter of a weaker or more unpredictable America could make Japan feel cornered—and become dangerous.