President Trump must put Philippine President Rodrigo Duterte on notice.

Trump should make clear that while the U.S. greatly values its longstanding alliance with the Philippines, that alliance cannot continue at the expense of America's interest in upholding its liberal international order against China.

The warning is necessary in light of Duterte's continued deference to China. A self-proclaimed strongman, Duterte is taking a pathetic stance in face of escalating Chinese aggression. As the Navy Times reported this weekend, Xi used Duterte's visit to China last week to tell him that China has no intention of abiding by a 2016 Hague tribunal ruling on sovereign rights in the South China Sea. China lost that ruling in the Philippines favor.

If Duterte were a resolute leader in service of his nation, you would think he might have reacted rather more sternly to Xi's arrogant rebuke. Think again. Instead, Duterte's spokesman offered that which Xi most adores — feudal submission.

"Both President Duterte and President Xi agreed," the spokesman said, "that while their variant positions will have to remain, their differences however need not derail nor diminish the amity between the two countries."

And Duterte says he's a strongman? Give me a break.

This pathetic submission isn't entirely surprising. Duterte has personal reasons to be a Chinese puppet. As the Asia Times reports, China's significant investment projects in the Philippines include some "on Duterte’s home southern island of Mindanao." Enrichment is a powerful motivator to politicians of less honorable purpose.

As I say, however, the U.S. cannot sit idle in face of Duterte's duplicity. The Trump administration has rightly pledged to defend the Philippines against external aggression. That sits with an American friendship of durability and significant economic, political, and military value. But if Duterte thinks he can have American cake and surrender U.S. international order to China, he should be educated to his error. Trump should make clear that alliances require reciprocity.

If Duterte wants to move closer to Beijing and make his nation into the next Hong Kong, that is his right, so long as FIlipinos keep electing him. But he must know that such a choice will invite American reassessment of our relationship with Manila. America has other allies such as Thailand, and increasing partners such as Vietnam, which have the courage to oppose Xi's imperialism.

If Duterte keeps this up, then perhaps the Vietnamese, our old enemy, will merit a warmer American embrace than the Filipinos, our longtime allies.