The thing to remember about President Trump’s threats in respect of Korea is the bitter lessons of history. The Korean war happened after, among other things, a famous — or infamous — speech by an American tribune. It left the communists with the impression that we might not view our vital national interests as extending to Korea. Before you could say Jack Robinson, we were at war.

Not only that but the speech was made by no less a Panjandrum than Secretary of State Acheson. He spoke in January 1950 at the National Press Club in Washington. One scholar, Jas. Matray, calls it “among the most important and controversial US policy statements in the early history of the Cold War in East Asia.” That’s because Acheson defined our “defensive perimeter” in the Pacific in a way that excluded Korea.

Six months later, the North Korean communists swarmed across the 38th Parallel and the war was on. And, Professor Matray writes in a 2002 article in the Journal of Conflict Studies, “more than fifty years after the start of the Korean War, countless South Koreans still hold Acheson responsible for igniting the fratricidal conflict.” They “bitterly maintain,” Mr. Matray writes, that America “committed an act of betrayal” toward Korea.

Not that Mr. Matray agrees with the South Korean critics of Acheson. The point of his article is to discuss the latest evidence. The release of Soviet documents in recent years, he writes, “has removed any doubt that North Korea planned and initiated the Korean War with the reluctant endorsement of Soviet leader Joseph Stalin.” The North’s tyrant, Kim Il Sung, began pressing the Kremlin to back an invasion in 1948.

More to the point, Mr. Matray presents what he calls “evidence from recently released Soviet documents that Acheson’s address had little if any impact on Communist deliberations.” Stalin, he writes, “worried about US military intervention until the moment the Korean War began.” Moreover, he adds, Old Joe “feared that North Korea could not survive an attack that he was certain South Korea would stage in the future.”

These columns have touched on the Acheson blunder before, but the episode beckons now more than ever. History reminds us that for President Trump, it is the part of wisdom to take no chances. Which seems to be the point of the president’s bluster. How Mr. Trump got that wisdom (and how the left wing press lost it) we will leave to his post-presidency. It may be that Mr. Trump is up on his Korean history. Or that it’s just his nature to speak in blunt terms. Either way, it’s better to err on the side of tough talk than to leave any doubt in Pyongyang, Moscow, and Beijing in respect of the consequences.