SEOUL, South Korea >> President Donald Trump took to Twitter following North Korea’s strongest-ever nuclear test explosion to criticize both Koreas and China. But his tweets will get as much attention in Asia for what’s missing as for their tough words.

Following the clearest sign yet that North Korea is fast approaching a viable arsenal of nuclear-tipped missiles capable of hitting the U.S. mainland, Trump again skipped what for decades has been the bedrock of U.S. policy on the Korean Peninsula: a firm assurance that the United States would defend South Korea against any attack.

This feeds a growing worry that has many in South Korea and Japan asking a startling question. Could Trump and North Korean leader Kim Jong Un want the same thing, namely a separation, or “decoupling,” of the decades-old security alliance between the United States and its top Asian allies, South Korea and Japan?

The White House has occasionally issued statements in which Trump has repeated what past presidents regularly declared about the U.S. commitment to defend its Asian allies, and he reportedly did so in a private phone call with South Korean Moon Jae-in nearly 36 hours after the nuke test. But his public comments on the alliance, which are what South and North Koreans hear, have more often reflected deep skepticism — and skipped any security reassurance.

Trump, for instance, previously questioned the expensive stationing of U.S. troops in South Korea and Japan, and suggested that Seoul and Tokyo pursue nukes themselves, instead of relying on the so-called U.S. nuclear umbrella. Trump also appears to be taking a shot at another pillar of the U.S.-South Korean alliance, a hard-fought free trade deal, by considering triggering a withdrawal from the agreement, a U.S. business lobbying group said over the weekend.

Then came Trump’s five tweets after the nuclear test which criticized North Korea’s main ally and aid provider, China, for failing to contain the North; South Korea’s liberal president for “talk of appeasement” (despite what many see as a consistent hard line toward the North’s weapons tests) and, of course, “rogue” North Korea.

Nowhere did he seek to reassure a frazzled South Korea that the United States would have its back if attacked.

This matters because North Korea’s relentless pursuit of nukes is seen by many analysts less as a way to beat the United States in a war than as a way to separate Washington from its Asian allies. The goal is to cause the United States to seriously consider whether it’s worthwhile to fulfill its treaty obligations by treating an attack on Seoul as it would an attack on San Francisco.

Ironclad U.S. vows of protection were easier before North Korea’s recent demonstrations that it may be very close to actually being able to hit San Francisco and other parts of the United States with nuclear missiles.

“What people in South Korea worry about most is whether the United States will defend South Korea at a time when the U.S. mainland is under threat (by North Korean missiles). If you look at what Trump said now, the answer seems to be no,” said Shin Hee-Seok, a graduate student in international law at Seoul’s Yonsei University. “While it still remains a fringe opinion, some South Koreans are wondering if we should now build our own nuclear deterrent. If the U.S. is not a reliable ally, South Korea may have to think about Plan B.”

The possibility of losing the free trade deal seemed for some here yet another hit to the alliance.

“The United States now is not the United States we used to know,” the Chosun Ilbo, South Korea’s largest daily newspaper, said in an editorial. “The president prioritizes dollars over the alliance.”

Some see a not-too-distant future where North Korea’s possession of dozens of nuclear-tipped ICBMs allows it to attack Seoul or Tokyo without U.S. intervention because of American fears that North Korean retaliation could kill millions in American cities.

“I’m worried about whether the U.S. is really serious about defending its ally, South Korea, or if it’s putting its own national interest first,” said Woo Young-soo, a law lecturer at a university in Seoul. “As a true ally, I wish Trump would have a defense policy that is truly meant for South Korea.”

Others believe North Korea can still be checked with firm statements from Washington that make clear how strongly the United States will respond if its allies are attacked. Because Kim Jong Un cares deeply about keeping power, this argument goes, he won’t risk an attack if an overwhelming U.S. response seems more likely than not.

Trump’s experienced lieutenants have sought to signal this. Defense Secretary Jim Mattis said all the right things after the test when he repeated what Seoul and Tokyo long to hear — that Washington’s commitment to them is unshakeable.

“A lot of reassurance comes down to trust,” Colin Kahl, a Georgetown University professor and former Obama administration national security official, said in a Twitter thread. “Our allies have to ‘believe’ we would trade San Francisco for Seoul or Toledo for Tokyo if push comes to shove. Yet instead of reassuring our democratic allies in East Asia, Trump has done the opposite.”

“Undermining alliance solidarity at this moment is dumb and dangerous. It emboldens Pyongyang, increases the risk of (North Korean) miscalculation (and) potentially incentivizes (South Korea) and Japan to seek their own independent nuclear arsenals,” Kahl wrote.