Just in time for President-elect Trump’s inauguration, a Class A crisis in South Korea. It saw President Yoon, leader of the free half of the peninsula and its conservative political party, declare martial law, only to be countered by the leftist opposition in the parliament, which voted unanimously to reject the demarche. History has taught us how close to the line the impassioned Korean republic is capable of taking these kinds of confrontations.

What strikes us about the current crisis is that not only is it a test of South Korea’s democracy, which, in our view, is better represented by President Yoon than by the larger opposition movement. The left, after all, has become infected by Chinese communist infiltrators and others who favor appeasement of communist North Korea. We are also struck by the test that this is going to represent for the incoming American administration of President Trump.

The president-elect’s posture toward the peninsula has been characterized by a mixture of bluster and a “rollercoaster relationship,” as Reuters puts it, with North Korea’s tyrant, Kim Jong-un. Trump likes to say that the two leaders “fell in love” over the course of an exchange of letters. Yet Trump was unable, despite three high-profile summits with Mr. Kim, to persuade Pyongyang to give up its nuclear arsenal.

Meanwhile, President Trump has taken to urging South Korea to compensate America more than it already does to cover the cost of defending the East Asian democracy. “If I were there now, they’d be paying us $10 billion a year. And you know what? They’d be happy to do it,” Trump said in October. He called South Korea a “money machine.” Since 1991, Seoul has been compensating America for some of its military expenses on the peninsula. 

“We have 40,000 troops in harm’s way, very serious, because you have North Korea’s very serious power. They have tremendous nuclear power,” said Trump. “I said to South Korea, ‘You’re going to pay,’ and they agreed to do it, and Biden then cut it back. And it’s a shame.” These remarks echo Trump’s call over the summer for Free China to compensate Uncle Sam for defending the island democracy. “I think Taiwan should pay us for defense,” he said.

The chaos that erupted today at Seoul, though, is a reminder of the fragility of democracy on the Korean peninsula and of America’s interest in preserving it. The divide between a free and prosperous South Korea and the tyrannical poverty-stricken regime in the North is testament to how, along with our allies in the South,  America has held the line against communist infiltration during the war on the peninsula that started in June 1950. 

The communists in the North — backed by the Soviet Union — had been emboldened when, some five months earlier, Secretary of State Acheson blundered by making a speech that raised doubts, by omitting South Korea from Washington’s Pacific “defense perimeter,” over America’s willingness to defend the regime at Seoul. That put South Korea among the lands that Acheson warned “no person can guarantee” against a “military attack.”

America’s intervention in Korea was a triumph in the twilight struggle against communism. While Trump’s call for our Asian allies to step up their defense commitments is a far cry from Acheson’s gaffe, his comments come amid concern at Beijing’s aggressiveness — and alarm at moves by Mr. Kim. President Trump can count the current crisis as an early warning of the need for good judgment and realism in a major theater of the Cold War.