WHEN a schoolyard is taken over by bullies, what are model pupils to do? Something like that quandary is now playing out in the global economy. Since returning to power in January, President Donald Trump has treated trade partners with the swaggering cruelty of a sixth-form tyrant. This marks a change from his first presidency when American officials acted as harsh disciplinarians. Back then, Trump aides called countries cheats for running trade surpluses with America. They demanded structural reforms from countries like China, accused of stealing American jobs and technologies by abusing world trade rules.