America is strengthening and deepening potential battle lines in Asia with deals and understandings reached this week between the defense secretary, Lloyd Austin, and two very different allies, South Korea and the Philippines.

To the outrage of North Korea, Mr. Austin and his South Korean hosts agreed to stage war games of increased intensity than past displays, including a full array of aircraft carriers and fighter jets. Then, Mr. Austin while in Manila succeeded in getting the Philippines to accept long-standing American pleas for regular access to four of its military bases, at least one of them facing the Chinese on the South China Sea.

Conspicuously missing from the defense secretary’s East Asian itinerary were Taiwan and Indonesia. Washington is firmly “committed” to the defense of Taiwan, the independent off-shore island province of China, but faces problems in Indonesia, a sprawling giant with which China has formed strong military and economic bonds.

It’s because China is aggressively pursuing close ties with countries all around its periphery that Mr. Austin’s swing through East Asia appeared as a tightly focused effort by Washington to build on alliances it’s already formed as bulwarks against Chinese aggression.

Possibly the most interesting of the deals struck by Mr. Austin and the Philippines president, Ferdinand “Bongbong” Marcos Jr., was the agreement giving American warships and planes access to a base on the long southwestern island of Palawan. The base is opposite the Spratly Islands in the middle of the South China Sea, where China has built major bases including landing strips and docking facilities. The Philippines also has several tiny islets in the area, but Chinese harassment makes them virtually useless.

Mr. Austin’s rapport with Mr. Marcos and the defense secretary, Carlito Galvez Jr., was all the more remarkable considering that Mr. Marcos has recently called on President Xi, telling him the Philippines would pursue “an independent foreign policy.”

That comment could be seen as either a promise not to resume tight ties with America or defiance of China’s inroads in the region. 

America ruled the Philippines as a colony after defeating the Spanish and then Philippine independence fighters at the end of the 19th century. America and the Philippines have been alliance partners since 1951, and America’s two largest overseas air and naval bases were in the Philippines until the Philippine senate refused to extend the deal 30 years ago.

America sought to regain limited access to the Philippines under an Enhanced Defense Cooperation Agreement reached in 2014, but Mr. Marcos’s predecessor, Rodrigo Duterte, downgraded ties with Washington while cozying up to Beijing. American influence in the Philippines runs deep, as Mr. Marcos made clear, remarking, “The future of the Philippines and for that matter the Asia-Pacific will always have to involve the United States.”

Mr. Austin’s mission to Korea and the Philippines, however, is sure to heighten tensions with China and North Korea. A Chinese spokesman accused America, “out of self-interest,” of continuing “to strengthen its military deployment in the region with a zero-sum mentality.” American moves, the spokesman said, were “exacerbating tension in the region and endangering regional peace and stability.”

North Korea unleashed the strongest vituperations, promising “nuke for nuke and an all-out confrontation for an all-out confrontation” against American forces. “We are seeing through the true intention of the U.S.,” the statement, carried in English by Pyongyang’s Korean Central News Agency, said, proclaiming “a clear counteraction strategy capable of coping with any short- and long-term scenarios.”

Mr. Austin’s remarks in both Korea and the Philippines edged America closer to armed conflict than any such missions in recent years.

In Manila, he said that America’s commitment to Philippine defense “applies to armed attacks on either of our armed forces, public vessels or aircraft anywhere in the South China Sea.” That could be interpreted to mean that American warships would counter Chinese patrol vessels whenever they menace or block Philippine fishing boats — gestures against which the Philippines has been virtually powerless.

In South Korea, Washington and Seoul saw themselves locked in a NATO-style embrace in which an attack on one would be considered an attack on both, with  each side treaty-bound to aid the other. That message lay at the crux of talks in Seoul between Mr.  Austin and his South Korean opposite number, Lee Jong-sup.

Mr. Austin, promising Washington would deploy more bombers and fighter planes for South Korea’s defense, declared in a message to Seoul’s Yonhap News that the North Koreans, “If they challenge one of us,” would be “challenging the U.S.-ROK alliance,” referring to the Republic of Korea. NATO’s Article 5 states that “an armed attack against one or more” NATO members “shall be considered an attack against them all.”

In what did not seem at all a coincidental move, the NATO secretary-general, Jens Stoltenberg, visited Seoul for the first time in more than five years. While there he reminded a conservative think tank, the Chey Institute for Advanced Studies, that “NATO’s history and our security has long been connected with yours.” NATO, he promised, would “stand with our partners in calling on Pyongyang to stop its provocations.”

In Japan, Mr. Stoltenberg told Prime Minister Kishida: “No NATO partner is closer or more capable than Japan,” while “Beijing and Moscow are leading an authoritarian pushback against the international rules-based order.”

Mr. Stoltenberg’s mission appeared to be carefully coordinated with Mr. Austin’s.  He implicitly endorsed Japan’s “defense strategy” for building up its armed forces — the focal point of Mr. Kishida’s White House summit with President Biden last month.