The impression created by the mission to Washington of President Yoon is that the South Korean leader has won much needed support for his country, our formal ally, at a time of rising threats from North Korea.

At the urging of President Biden during a dinner Wednesday, Mr. Yoon, a more hawkish figure than his predecessor, sang a light-hearted rendition of “American Pie.” Today, he delivered well-received remarks before Congress.

His audience in Congress, including four Korean-Americans — two from each party, as he observed — obviously loved his message beginning with the words, “No matter where you sit, you stand with Korea,” and ending, “God bless our alliance.”

The speech also raised the stakes in the confrontation with North Korea, whose leader, Kim Jong-un, has ordered scores of missile tests while repeatedly threatening the South with the use of nuclear weapons and refusing to negotiate denuclearization.

“There is one regime that has chosen to pursue the wrong path,” Mr. Yoon warned. “That is North Korea,” whose “nuclear program and provocations pose a serious threat.” “Along with the U.S. and Japan,” he said, “we must speed up to counter the North Korean nuclear threat.”

The wording was portentous for two reasons. First, Mr. Yoon did not mention the need for South Korea to develop its own nuclear weapons, as polls show a majority of Koreans favor but Washington opposes. Rather, he said that he and Mr. Biden had “agreed to expand deterrence,” for which South Korea would still count on America for the nuclear umbrella needed to counter the North.

Second, Mr. Yoon, by noting the need to get along with Japan, Korea’s historic enemy and former colonial ruler, acknowledged the importance of “trilateral” cooperation with Japan and America even though Korea and Japan, both allied with America, refuse to ally with each other.

As a sign of the rapport between Mr. Yoon and his American host, Mr. Yoon suggested South Korea was thoroughly aligned with America against the Russian invasion of Ukraine after having appeared almost neutral in the war.

“Freedom and democracy are once again under threat,” he said. “The war against Ukraine is a violation of international law. Korea strongly condemns the unprovoked attacks against Ukraine.”

Drawing the analogy of the North Korean and Chinese invasion of South Korea in the Korean War, he said “the democracies came to rescue us” in 1950. “In solidarity with the free world,” he vowed Korea “will work to promote freedom in Ukraine.”

Mr. Yoon did not say that South Korea, one of the world’s largest manufacturers of arms ranging from fighter planes and tanks to artillery shells and machine-gun bullets, would now export them directly to Ukraine. Having already sent artillery shells to Ukraine via Poland and America, Korea may remain undecided as to the degree to which it’s willing to oppose the Russians in the war.

Yet would Korea ally with America in what has evolved into a defensive line against China under the rubric of Aukus, the grouping of Australia, the United Kingdom and the U.S., and Quad Four, including America, Australia, India, and Japan?  

Carefully, Mr. Yoon said not a word about the Republic of China on Taiwan, to which Mr. Biden has said America has a “commitment” even though it acknowledges only “one China,” the People’s Republic of China under Communist rule. Nor did he mention the South China Sea, which Beijing claims as part of China over the strong objections of Washington.

Instead, Mr. Yoon stuck to generalities rather than offend China, the South’s biggest trading partner. “Korea is committed to fostering freedom and prosperity in the Indo-Pacific,” he said. “We will take a comprehensive approach.”

Mr. Yoon also delivered what could be seen as a stern rebuke to a proposal in the lower house for a law calling for a peace treaty formally ending the Korean War, which closed with an armed truce on July 27, 1953, and the formation of the alliance between America and South Korea that’s lasted for 70 years.

Korea strongly opposes the bill, which has already collected the signatures of several dozen members of Congress, as a ruse to get America to withdraw its remaining 28,500 troops from Korea and break off the Korean-American alliance.

“Disinformation threatens democracy and the rule of law,” he said. “We must not be fooled by such deception. We have protected democracy and the rule of law with our blood and threat.”

Mr. Yoon’s remarks were repeatedly interrupted by applause, never more so than when he said, “Together with the U.S. Korea will play the role as a compass for freedom” ready to “safeguard and broaden the freedom of citizens of the world.”