Asia Pacific Economic Cooperation nations, staring down U.S. tariffs, jockeyed for time with U.S. Trade Representative Jamieson Greer at a gathering in South Korea.

No part of the world will be hit harder by President Trump’s barrage of “reciprocal” tariffs than the Asia-Pacific region. Despite his 90-day pause in imposing them, some of Mr. Trump’s steepest tariff rates still hang over developing economies such …