German Chancellor Angela Merkel took aim at Donald Trump’s world view in a speech on U.S. soil, saying unilateralism risks bringing down post-World War II alliances and trade wars threaten the “foundations of our prosperity.” “More than ever, our way of thinking and actions have to be multilateral rather than unilateral,” Merkel said in a commencement speech to Harvard University’s class of 2019 that combined tough criticism of nationalist policies with philosophical vistas of a change for the better. Merkel, 64, was addressing a receptive audience on Thursday as she seeks halt a deterioration in the trans-Atlantic alliance under Trump’s...