As Asian strongmen sound siren songs of “peaceful” unification with Pacific neighbors, President Trump should resist his inner isolationist impulses and make sure everyone knows America will ­defend our allies.

In separate New Year speeches, President Xi Jinping of China talked about Beijing’s long-held desire to annex Taiwan, while North Korea’s Kim Jong Un advocated unity with his democratic neighbors in South Korea.

“Reunification is the historical trend and the right path,” said Mr. Xi, warning that any notion of Taiwanese independence is “a dead end.” Mr. Kim, meanwhile, spoke of ending “military hostility between North and South” and transforming the Korean Peninsula into “a durable and lasting peace zone.”

On Wednesday, Taiwan’s presiden, Tsai Ing-wen, rejected Mr. Xi’s offer to unite her country with Beijing under the “one country, two systems” formula. But Ms. Tsai’s dovish South Korean counterpart, Moon Jae-in, has adopted that formula in recent speeches, following the cues of Messrs. Kim’s and Xi.

My sources in Taiwan, a country that fears Chinese invasion, laugh bitterly. After all, they say, Hong Kong was promised “one country, two systems” when China took over in 1997. Now the final traces of Hong Kong’s democratic freedoms are disappearing under pressure from the mainland.

Yet in his eagerness to end the state of war, South Korea’s Moon is enthusiastically promoting joint industrial, artistic and athletic projects with the North. Will a unified Korea look more like the prosperous and free South, or like the darkened, tyrannical North?

Ask Mr. Kim. In his annual New Year speech, the scion of the slave-state dynasty creepily talked about enforcing “moral discipline” in North Korea to promote a “harmonious family filled with moral ­excellence and tender feelings.”

Such talk may have spooked Jo Song-gi, the North’s ambassador in Italy, who apparently defected this week. The prospect of Mr. Kim extending his “harmonious family” south of the 38th parallel should also terrify South Koreans.

So why haven’t the Kims, who boast the world’s fourth largest army, already crossed that armistice line?

David Maxwell, a Korea watcher at the Foundation for Defense of Democracies, says that Pyongyang officials almost uniformly point to American troops stationed on the Peninsula. True, America’s 28,000 troops “aren’t going to defeat North ­Korea,” Mr. Maxwell says, “ but they deter it from invading.”

Yet Mr. Kim surely noticed that Mr. Trump recently pulled American troops from Syria after a chat with Turkish strongman Recep Tayyip Erdogan. So what is to stop Mr. Trump from doing the same after talking with Mr. Moon?

Washington’s conversation with Seoul about upping the South’s financial contribution for the troops’ upkeep has lately stalled. And remember: Military burden sharing is a signature Trump issue. Mr. Trump might be tempted to think his preliminary diplomacy means Pyongyang is ready to disarm.

Wrong. As Tufts University Korea expert Sung-Yoon Lee tweeted Wednesday, Pyongyang’s official English translation of Mr. Kim’s New Year speech quoted him as promising “complete denuclearization.” It omitted the rest of Mr. Kim’s sentence in the original Korean: “. . . of the Korean Peninsula.”

Mr. Kim may be luring America into believing he is ready to disarm unilaterally, while telling his people he would — only if the US does.

By heightening anxieties about a North Korean or Chinese invasion into neighbors’ territories, Trumpian withdrawal signals in the ­Pacific could threaten Asia’s world-leading economies — and have ­extremely negative effects on our economy, as well.

True, Mr. Trump’s diplomacy so far succeeded in halting Pyongyang’s nuclear and ballistic-missile tests. In his Tuesday speech, moreover, Mr. Kim promised to quit arms proliferation as well. That’s progress. The North has made rosy promises before — only to brazenly violate them.

The whole region is tense, as witnessed by Thursday’s State ­Department warning to Americans traveling to China. For too long we have taken a peaceful Pacific region for granted and lost sight of the one force that has made that relative tranquility possible: American military commitments to allies.

So Trump better broadcast widely that he doesn’t intend to ­reduce our military presence in Korea or wobble on our vow to defend Taiwan.

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This column first appeared in the New York Post.