Hillary Clinton’s famous “pivot to Asia” never made the turn needed for Barack Obama’s administration to face up to the threat to Asia, which is China’s expanding regional aggression. China’s mammoth size, of both territory and people, makes it the heavy. “When China spits,” the ancient wisdom goes, “Asia swims.”

The people who swim there have shown due skepticism of Mr. Obama’s commitment to his pivot, as executed by Mrs. Clinton, especially in view of their growing Chinese commercial ties and their over-the-shoulder look at Beijing’s expanding navy.

Relations between Washington and Bangkok have soured since the May 2014 coup in Bangkok, another in a history of military takeovers and the second in a decade. The subsequent crackdown by the junta, encouraged by China, has strained U.S.-Thai relations. The Thais have thanked China with gestures, such as sending back to Beijing several human-rights exiles, and participating in joint military exercises.

Pundits who have just walked into the conversation raise alarm over the Chinese competition to the strong U.S.-Thai alliance, crucial since the early days of the Cold War and particularly during the war in Vietnam, when the United States maintained several large air bases in Thailand.

Bangkok’s flirtation with China can be seen as the 21st century version of an old game, playing both sides to maintain independence. But it’s difficult now because Thailand has emerged as one of the strongest economies in Southeast Asia.

There is new danger, too, in expanded communications ― including rail and road access from China and neighboring Laos, new “geography” for a region once accustomed to Chinese emigration only by sea. In that earlier time the Sino-Thai elite emphasized their native customs and traditions, and that, too, is more difficult now, when it’s more profitable to embrace China.

Everything is more difficult now with the government in Washington determined to “lead from behind.” It has little skill in playing a traditional regional role of supporting local nationalism and freedom of navigation. So the Obama administration stumbles along, trying to make the promised pivot.