The world’s major growth engines are about to run in reverse. The policies and uncertainties of US President Donald Trump’s second administration have hit a sluggish global economy with a transformational exogenous shock. Risks are especially worrisome in both the United States and China, which have collectively accounted for a little more than 40% of cumulative global GDP growth since 2010.

America is now the problem, not the solution. Long the anchor of the rules-based international order, the US has turned protectionist, posing major risks to an already fragile global trade cycle. At the same time, Trum...