This massive geopolitical shift is entirely Duterte’s doing. It cannot be explained any other way. It is a product of his peculiar psychology.

He has long been ideologically hostile to the United States — he has called Obama a “son of a whore” — and he feels an ideological affinity with China’s authoritarian rulers. Although elected democratically, Duterte is a strongman in the making. He has already violated the rule of law to unleash death squads that are said to have killed at least 1,900 people, including a 5-year-old boy, in the name of fighting drugs. He has cited Hitler as his role model: “Hitler massacred 3 million Jews. Now, there is 3 million drug addicts. I’d be happy to slaughter them.” He has also said “I don’t give a shit” about human rights. China’s rulers don’t put their worldview quite so crassly, but they, too, don’t care much for human rights. The Duterte-Xi Jinping marriage thus seems like a natural match.

From the American viewpoint, Duterte’s flip-flop — assuming it leads to a lasting strategic shift — is a potential disaster. Aligned with the United States and its regional allies, the Philippines can provide a vital platform to oppose Chinese aggression in the South China and East China seas.

If the Philippines becomes a Chinese satrapy, by contrast, Washington will find itself hard-pressed to hold the “first island chain” in the Western Pacific that encompasses “the Japanese archipelago, the Ryukyus, Taiwan, and the Philippine archipelago.” Defending that line of island barriers has been a linchpin of U.S. strategy since the Cold War. It now could be undone because of the whims of one unhinged leader.