Thousands of US companies opened for business on Friday with no idea whether they had to pay tariffs. …In imposing the tariffs, Mr. Trump misused the 1977 International Emergency Economic Powers Act. Congress passed IEEPA to give presidents tools to respond quickly to emergencies like wars or terrorist attacks. …IEEPA has never been used for tariffs, and courts should rule that it can’t be. …US trade laws require the president to identify and investigate specific trade practices that harm the U.S. and then propose remedies, such as tariffs or negotiations. A president may impose tariffs on specific goods if fact-finding determines that imports threaten US national security. Mr. Trump’s reasons for imposing tariffs on Canada don’t appear to satisfy these standards. …Faced with Mr. Trump’s tariff threats, Canadian, Mexican and European officials have indicated that they are open to collective action against China, the world’s biggest exporter and a major source of US trade deficits. [to access the full story, a WSJ subscription is required]