The big news in respect of North Korea is not so much the communists’ claim to have lit off an H-bomb but President Trump’s warning to South Korea in respect of appeasement. “South Korea is finding, as I have told them, that their talk of appeasement with North Korea will not work, they only understand one thing!” is the way the president put it in a message sent out on the internet Sunday.

We can’t recall a previous president having used that kind of language in cautioning our South Korean ally. And we don’t mind saying that we share Mr. Trump’s concern. He was referring to the overtures to the North from the new government in the South, which the Sun has warned about in “Who Lost Korea?”. That editorial was issued in May, after the leftist human rights lawyer Moon Jae-in was elected president on a campaign calling for, among other things, an entente with the communist north.

In another editorial, “Korean Kapitulation?”, we warned about the talks with North Korea in which Americans were participating at Oslo. The parley hit the news in the wake of a statement by Mr. Trump that he’d be willing to meet with the North Korean dictator, Kim Jong Un, “under the right circumstances.” We came up in an earlier era, when the idea of American direct talks, even unofficial, with the North Korean regime would have been unthinkable.

This goes all the way back to the Armistice Agreement that was inked in 1953. By our lights, opponents of that parchment — including President Syngman Rhee — look ever more farsighted. The armistice anticipated peace talks. They failed at Geneva in 1954. North Korea’s military buildup, in violation of the armistice, prompted our side in 1956 to abrogate paragraph 13(d) of the armistice and introduce to the theater atomic cannons and the Honest John.

Whatever else one can say about the Korean epic since then, it’s hard to identify a single successful parley with any element of the communist regime in the North. The bottom line is that it is our side that has established a real democracy, with a free press, religion, property, and travel. Through it all, appeasement of various sorts has been pursued by Presidents Clinton, Bush, and Obama. And now the North is launching strategic missiles and is claiming to have tested an H-bomb.

So it would be nice to think that Mr. Trump’s weekend tweet warning about appeasement represents a considered change of course. It is sometimes said that there is no military solution in Korea. Beware of talking, we say. It is the bigger danger. It incents the type of testing — of atomic weapons and of our resolve — the North Koreans are now pursuing. The only thing the communists understand is military power. The fact that it’s a cliche doesn’t mean it isn’t true.