The president’s trade assault, which makes no distinction between ally and adversary, is an assertion of U.S. dominance with significant risks.

Amid warnings of price increases, Canada moved quickly to retaliate, China said it had planned countermeasures and Mexico said it would soon unveil its response.

President Donald Trump said Sunday that Americans could feel “some pain” from the emerging trade war triggered by his tariffs against Canada, Mexico and China, and claimed that Canada would “cease to exist” without its trade surplus with the United States.