What a tumler, that Kim Jong Un, or, as President Trump once called him, “Rocketman.” The North Korean dictator is threatening to send troops into the demilitarized zone that separates communist tyranny from the free Korean republic to the south. And, in a bizarre display, the communist regime blows up a two year old inter-Korean liaison office on its own side of the line. Just in time to mark next week’s somber anniversary.

On June 25 it will be 70 years since Kim’s grandfather, supported by Mao Tse-tung’s China and Joseph Stalin’s Russia, sent his troops across the 38th parallel and launched the Korean War. America’s military response took place under the auspices of the United Nations. Most of the active campaigning occurred in the first twelve months; after a subsequent two year stalemate the conflict was halted in 1953.

The armistice — not a permanent peace — preserved the territorial integrity of free Korea at the cost of 36,000 Americans killed in action. They fell a long way from home. Korean deaths, including civilians, were far greater. An exceptionally bloody conflict, the war claimed a total of five million lives, including 10% of the peninsula’s civilian population.

It’s hard to argue those Americans died in vain. South Korea, where America has maintained troops for seven decades, is today a flourishing democracy. It’s the 12th largest economy in the world. It boasts a per capita GDP greater than the European Union’s. The communistic North Korea remains a totalitarian wasteland and a threat to international peace.

The war itself was also a watershed moment in the evolution of postwar American foreign policy, with implications far beyond Northeast Asia. We sometimes forget that in 1945, a fledgling Truman administration, following Roosevelt’s Yalta script, exhibited wishful thinking and naive expectations regarding co-existence with the Soviets, and hopes that Chiang Kai Shek’s Chinese Nationalists might survive the civil war with Mao.

Our famed undersecretary of state at the time, Dean Acheson, actually believed that Soviet Russia was a “great ally upon our cooperation with whom rests the future peace of the world.” Acheson even endorsed the idea of sharing America’s atomic weapon technology with the Soviets, a scheme cooked up by Henry Stimson, the departing Secretary of War. We used the proper term back then.

Those attitudes began changing in 1946 with George Kennan’s outline of the policy of containment and in 1947 with the proclamation of the Truman Doctrine and in 1948 with the implementation of the Marshall Plan, and in 1949 with signing of the North Atlantic Treaty. Events like the coup the Kremlin directed in 1948 in Czechoslovakia and subsequent blockade of West Berlin contributed to the shift to a harder line in Washington.

Nonetheless, no one in the administration had put together a comprehensive statement of what America’s national security should be, and there were no plans underway to bulk up our military capabilities. Under pressure from the Taft wing of the Republican party, and then-current economic orthodoxy, Truman was looking to limit postwar military spending and reduce a national debt that exceeded our GDP.

An opponent of the North Atlantic Treaty, Kennan and his planning group at State conveniently argued that “it is not Russian military power which is threatening us, it is Russian political power.” The North Atlantic Treaty Organization initially functioned more as a political alliance than a military pact with uncertain prospects for funding (plus ca change, eh?).

In Asia the focus was also on defining limits. Viewing a Maoist regime as a minimal threat, America stood by in 1949 as Chiang’s Chinese Nationalists were defeated, and Acheson — by now secretary of state — suggested waiting “for the dust to settle” before taking any steps directed at the new regime. He was soon to make one of the most fateful blunders in the annals of our diplomacy.

That came in January 1950 in a speech at the National Press Club in Washington, where Acheson announced a Pacific “defensive perimeter” strategy that would include American guarantees for Japan, Okinawa, and the Philippines, but exclude commitments to defend South Korea or Chiang’s precarious new base on Formosa. Unfortunately, as John Lewis Gaddis points out, Stalin, Mao, and Kim grandpere read his speech.

In January, 1950 Truman authorized the development of a hydrogen bomb. The Soviets had lit off their first fission bomb in September. The long postponed discussion of national defense strategy began in earnest. In April, Paul Nitze, George Kennan’s successor as State Department planning chief, wrote a memo known as NSC-68. It outlined in detail the urgent need for an American buildup of both conventional and atomic weapons to confront the Soviets and their allies wherever necessary.

The cost, in today’s policy terms, would entail “whatever it takes.” At that juncture, even with anti-communits sentiment on the boil in Washington, Truman nursed doubts that increased appropriations would pass congressional muster. In the 1948 campaign, he had pledged to cap the defense budget at $15 billion, a paltry sum compared to defense outlays of $100 billion in the last year of World War II. Truman was loath to backtrack.

The outbreak of war in Korea changed all that. An August Gallup poll indicated 70% of Americans would accept higher taxes to fund increased military spending. For political as well as patriotic reasons, the Republicans now swung behind a more robust defense, casting the Administration as soft on communism. Calls of “Who Lost China” echoed on Capitol Hill. By the 1952 campaign, Senator Nixon was describing Adlai Stevenson as a man with a “Ph.D. from Dean Acheson’s cowardly college of Communist containment.”

In September, Truman approved the NSC-68 blueprint. When the Chinese intervened in the war on November 28, any notions of spending limitations were swept away. By the war’s end in 1953, defense spending had quadrupled to $60 billion from 1948’s level and tripled as a percent of our soaring GDP. When it came to the long term funding of our defense, Dean Acheson said, “Korea saved us.”

Korea also put paid to any of illusions brought home from Yalta. American policy was now to be based on what Acheson termed “situations of strength.” The enlarged defense establishment became permanent; NATO was funded as a real military alliance under an integrated Yankee command. Korea took containment beyond its origins as a European strategy to one that would include Asia.

The Truman administration had finally “grasped,” as one historian, Wilson Miscamble, put it, “the essential world realities and assumed the demanding responsibilities of international leadership.” It turned out to be a leadership that would endured through victory in the Cold War and under presidents of both parties, until a reappraisal was demanded by, in President Trump, a new kind of Republican.

In 2016, Mr. Trump demanded a reappraisal of exactly what America’s responsibilities should be in Europe and Asia, not to mention the rest of the world. After three years, the results are inconclusive, and the question is likely to be taken to the hustings in the coming months. What a moment to remember what Harry Truman learned seventy years ago — that unexpected crises can be the spur toward a more coherent outlook.

And also something Dean Acheson said in his years of reminiscence. The job of a statesman seeking to “gain support for a major policy is not that of the writer of a doctoral thesis,” Acheson argued. In selling a new vision, he suggested, the part of the statesman was to opt instead for “bluntness, almost brutality.” It’s something for everyone to think about, including “Rocketman.”

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Mr. Atkinson, a contributing editor of the Sun, covers the 20th Century. Image: Drawing by Elliott Banfield, courtesy of the artist.