The Justice Department’s antitrust division, a competition-law enforcement unit, was told earlier this week to stop all cooperation with Canadian authorities on cases and policy matters. The directive came from Lynda Marshall, chief of the division’s international section, in a Wednesday email titled "Pause on Canada," reviewed by The Wall Street Journal. Marshall gave no explanation and said she would "circle back if the guidance changes," making the order the latest development in the escalating trade dispute between the two countries.