AMID A Capitol Hill impeachment drama arising from tensions between President Trump and a relatively new U.S. ally across the Atlantic, Ukraine, differences were also growing between the United States and an old ally across the Pacific: South Korea. The source of friction, not just coincidentally, is Mr. Trump’s view that supporting traditional allies is a bad “deal” for Americans, and that ungrateful proteges need to pay up.

This past week, Mr. Trump sent negotiators to seek a larger South Korean contribution to the costs of keeping 28,500 U.S. troops in that country, from $923 million per year, the current level, to a reported $5 billion. When Korean counterparts balked at this demand — highly unpopular in their country — Mr. Trump’s envoys walked out. Media outlets in Seoul reported that Mr. Trump was considering raising the pressure by pulling out 4,000 troops; Defense Secretary Mark T. Esper denied it, though the fact remains that Mr. Trump himself has spoken of a pullout previously. Meanwhile, South Korea set up more military hotlines with China and promised to “foster bilateral exchanges and cooperation in defence,” starting with a visit by South Korea’s defense minister to Beijing next year. This is more likely a South Korean pressure tactic for the talks with the United States than the beginning of a Seoul-Beijing alliance, but it also must be seen as a hedge against an increasingly less reliable United States.

Unquestionably, it is worrisome. The United States has maintained a troop presence on the Korean Peninsula since the end of the Korean War. Thanks in part to that commitment, northeastern Asia has been spared a new war, while South Korea has grown into a democracy and the 11th-largest economy in the world. It’s the second attribute, wealth, that impresses Mr. Trump most: The cost of keeping troops in a prosperous nation has rankled him for years, especially in light of the trade surplus South Korea enjoys with the United States ($17.9 billion in 2018).

No doubt, South Korea could afford to pay more. (Its current share amounts to about 40 percent of day-to-day expenses.) Indeed, Seoul’s 2018 payment represents an 8 percent increase over the previous year. Yet Mr. Trump’s attitude totally discounts the fact that the South Koreans paid 90 percent of the $10.8 billion construction cost for an immense new U.S. base 40 miles south of Seoul. South Korea’s own male population faces obligatory military service. Mr. Trump seems not to comprehend that a forward defense position in Asia pays for itself, in security.

Even as he makes demands on the democratic U.S. ally in Seoul, Mr. Trump adopts a supplicant posture toward the totalitarian U.S. foe in Pyongyang, most recently via his postponement of joint U.S.-South Korean military exercises and his implied summit meeting request to North Korean dictator Kim Jong Un on Twitter. The tweet, which included a childish insult of former vice president Joe Biden, ended “See you soon!” Mr. Kim — a genuine expert in extortion — snubbed Mr. Trump.

The president’s shortsighted policy represents anything but the “steadfast and strong” support for South Korea he promised before his inauguration. In fact, everywhere you look in northeastern Asia, American steadfastness is in doubt, and so is American influence.

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