The Asia Reassurance Initiative Act, whose name boldly proclaims its intent, includes a requirement that the president justify any sanctions relief to North Korea before Congress. And the National Defense Authorization Act for 2019 bars using any military funding to reduce American forces in South Korea below 22,000, unless the secretary of defense can certify that allies have been consulted and that the reduction won’t harm their, or America’s, security.

Still, Mr. Trump’s mercantile approach to stationing American troops abroad is highly detrimental to America’s role in the world, and to its own security and prosperity. In effect, he reduces the Americans abroad to a for-profit mercenary force, willfully ignoring the fact that they have been stationed in South Korea since the end of the Korean War not only to protect the South Koreans, but as the front line of the free world. Their presence affirms America’s position as a bulwark against the ambitions of suspect actors, including China, a role that pays concrete dividends in peaceful trade. How the waning of American influence could play out was demonstrated by an announcement Sunday that the defense ministers of South Korea and China had signed an agreement on improving their security ties.

And South Korea is not getting a free ride, even by Mr. Trump’s abacus. Seoul pays almost half the cost of maintaining American troops in South Korea and spends much of its arms budget in the United States. Those same troops would cost far more to maintain on bases in the United States, and their duties in South Korea provide real-world training that exercises at home could never duplicate.

Perhaps most damaging, Mr. Trump’s unreasonable compensation demand undermines yet another alliance. South Koreans have consistently shown overwhelming support for their alliance with the United States, whether under dovish or hawkish presidents. Mr. Trump’s demands, however, have provoked outrage.

There is never a good time to squander political capital or weaken alliances, but this is an especially bad one in Asia. For all the hype about Mr. Trump’s meetings with North Korea’s dictator, the North is continuing to test missiles, while China is rapidly growing as a powerful and ambitious rival in Asia. And South Korea is entangled in a nasty feud with America’s other major Asian ally, Japan. Not surprisingly, Mr. Trump is also demanding that Japan fork out four times more than it has for American troops, though those negotiations are still some time off.