Many Americans are persuaded that trade agreements of the last 40 years have been a costly mistake. To be sure, open markets leave some people behind, and promises to help those hurt were broken. But on balance, expanding trade has greatly benefited the US economy. More must be done to help those harmed, but protectionism is not the solution, write Alan Wm. Wolff, Robert Z. Lawrence, and Gary Clyde Hufbauer.