U.S. leadership of the global trading system has helped stabilize an area of international life that had bred conflict, even war, for centuries. To abdicate in favor of Mr. Trump’s zero-sum mind-set not only would undo the work of generations and lower the United States’ standing among the nations; it also would license other nations to conduct themselves just as selfishly. The disruption to market confidence could breed economic damage in excess of any transitory benefits. His approach would be a historic error, which, as president, he would be free to commit.
It's a little wonky, but you can read the rest at the Washington Post.