SEOUL — Defense Secretary Hegseth is leaving no doubt about it: President Trump is as firmly “committed” as was President Biden to the defense of America’s Asian allies — and also the free island province of Taiwan that China’s President Xi has vowed to recover by 2027.

In stops in the Philippines and then in Japan, Mr. Hegseth has talked tough and unequivocally about the need to stand up to Communist China, just as his boss is doing in imposing steep tariff increases beginning Tuesday.

Winding up his first overseas trip as defense secretary in Tokyo, standing beside Japan’s defense minister, Gen Nakatani, Mr. Hegseth declared, “America is committed to sustaining robust, ready and credible deterrence in the Indo Pacific, including across the Taiwan Strait.” Japan, he said, “would be on the front lines of any contingency we might face in the western Pacific and we stand together in support of each other.”

That unequivocal assurance of America’s commitment was clearly intended to remove recurring questions as to whether Mr. Trump would be willing to go to war for Taiwan, for which Congress last year approved the sale of $1 billion in arms and equipment. If nothing else, Mr. Hegseth sought to dispel worries that Mr. Trump might prefer to appeal to America’s foes, notably President Putin and North Korea’s leader Kim Jong-un, while abandoning allies and friends, such as South Korea and Ukraine.

“We have a robust alliance agenda that will strengthen our deterrence posture, keep the enemy guessing, creating dilemmas for them,” he said. Then, in a specific rebuff to China, he said America’s renewed commitment also meant “expanding access to key terrain in the first island chain, such as Japan’s southwest islands” — from the southernmost Japanese prefecture of Okinawa where American marines and air force planes are based, and extending to Taiwan and the South China Sea, claimed by China.

In one brief sentence, Mr. Hegseth hyped-up the possibility of more elaborate joint military exercises in defiance of China anywhere from Okinawa to the South China Sea where China has built naval and air bases on atolls and islets. Achieving that goal, he said with understated menace, “means exercising together at those critical locations.” To make it happen, America has begun “upgrading U.S. Forces Japan to a Joint Force Headquarters,” increasing “our readiness to respond to contingency or crisis” and “help Japan and U.S. forces defend this territory.”

Mr. Hegseth did not mention that the upgrade of the bond was initiated under the Biden administration. Nor did he refer to the alliance of “Australia, United Kingdom, and the U.S.” known as AUKUS, formed under Mr. Biden, but his remarks were the clearest affirmation yet that Mr. Trump might go further than Mr. Biden in defending an arc running from South Korea and Japan to the Philippines and on down to Australia and New Zealand.  

The downside of Mr. Hegseth’s first overseas mission as defense secretary was that, like his predecessor, Lloyd Austin, he had to skip South Korea, ordinarily a “must” stop on top-level American missions to the region. Koreans suspect Mr. Hegseeth did not want to compromise Mr. Trump’s stated desire to renew acquaintances with Mr. Kim, with whom he has said he “fell in love” in their summit in Singapore in June 2018, but South Korea’s ongoing leadership crisis is to blame

Washington does not want to take sides or interfere in South Korean affairs while the constitutional court considers whether to approve or dismiss a motion passed by the national assembly to impeach the South’s conservative president, Yoon Suk-yeol. The court’s eight sitting justices are reportedly at odds with one another while the government is in the hands of the acting president, Han Duck-soo, a former ambassador to Washington who was prime minister when Mr. Yoon was impeached.

Mr. Hegseth, before leaving Washington did say Americans needed to stand “shoulder to shoulder” with their Korean as well as Japanese and Filipino allies but, to the chagrin of South Koreans, barely mentioned Korea in the Philippines and then Japan. It was up to Mr. Nakatani to affirm “multilateral cooperation with regional partners, including Australia, the Republic of Korea and the Philippines.”