SEOUL – The blow-up of President Trump’s meeting with President Zelensky is sending shockwaves through East Asian countries fearful of the loss of America’s commitment to defend them against the region’s two giants, Communist China and Russia.

The “explosive encounter” in the Oval office between Messrs. Trump and Zelensky “has raised rich opportunities” for Communist China’s party boss, Xi Jinping, said the Australian Financial Review, “as China increasingly flexes its military muscle in the Asia-Pacific region.”

The danger of loss or weakening of American military resolve might appear even greater on Asia’s eastern periphery than in Europe, where at least two member nations of the North Atlantic Treaty, Britain and France, are seeking to put the quest for peace back on track — or defend Ukraine with or without Washington’s support.

For western Europe, the concern is not whether President Putin will attack them directly but whether they should let him win a war in Ukraine that could tempt him to try to recover other east European countries that once were part of the Soviet empire.

From South Korea to Japan, Taiwan and the Philippines, the question is how Mr. Trump would respond to any attack on them after years of armed confrontation. North Korea’s nukes and missiles intensify the threat since its leader, Kim Jong-un, has forged an alliance with Mr. Putin to whom he’s shipping arms and troops to fight Russia’s war against Ukraine.

Mr. Trump “says Kim Jong-un is a friend,” Kim Sung-gook, a Presbyterian pastor, told the Sun at a demonstration of several hundred thousand people waving American and South Korean flags on the anniversary of an uprising on March 1, 1919, in which Japanese forces massacred thousands of Koreans protesting Japanese rule. “If he withdraws American troops,” as he threatened to do during his first presidency, “we will no longer be a country.”

North Korea plunged into the Ukraine War in October by sending about 12,000 troops, supplemented last month by several thousand more in place of those killed or wounded in their first weeks of combat. The North has about 1.2 million troops in all services, more than half within striking distance of the demilitarized zone between North and South Korea.

In an interview with the Korea Times, a researcher at the Korea Institute for Defense Analyses, Doo Jin-ho, said the war “has given North Korea a rare chance to modernize its outdated conventional weapons.” The South’s armed forces, he noted, have had “no facto real-world combat experience since the Vietnam War.”

The precedent set by Mr. Trump’s call for a deal on Ukraine may have worse implications for the Republic of China, on Taiwan, which China regularly surrounds with ships and warplanes.

In response to expansion by Free China of military exercises to counter non-stop threats from Beijing, a Communist Chinese military spokesman warned, “We will come and get you, sooner or later.” A Republic of China Coast Guard vessel recently chased a Communist Chinese ship from Taiwan waters, and a Japanese destroyer cruised the Taiwan Straits between Taiwan and the Chinese mainland in a show of defiance.

Communist China, like Russia, has to be encouraged by the rift between Messrs. Trump and Zelensky, the latter of whom objected to any deal without guarantees to prevent Mr. Putin from violating the terms.

Mr. Trump has waffled on America’s commitment to defend Taiwan, an independent Chinese province — and China’s only democracy — with which Washington has no formal diplomatic relations. Demanding Taiwan repay Washington for billions of dollars in weapons, Mr. Trump avoided comment when asked if he would defend the island.

“I never comment on that,” he said. “I don’t ever want to put myself in that position.”