Bangladesh, Sept. 14 -- For years, US President Donald Trump's most dramatic attacks on traditional American allies have been directed at Europe. But Washington's increasingly transactional approach to alliances is no longer confined to the Atlantic. Asia is now getting a taste of the same medicine-and South Korea may be the first major casualty.

Trump's abrupt decision last month to cut short the annual "Ulchi Freedom Shield" military exercises with South Korea sent an unmistakable signal to Seoul. The timing and manner of the decision were particularly damaging because the exercises are not merely routine military drills. They are a highly visible demonstration of the US commitment to defending South Korea against North Korean aggress...