WASHINGTON— President Donald Trump on Saturday signed an order to impose stiff tariffs on imports from Mexico, Canada and China, drawing swift retaliation and an undeniable sense of betrayal from the country's North American neighbors as a trade war erupted among the longtime allies.

British Prime Minister Keir Starmer said Sunday that he would seek a strong trade relationship with the U.S. after President Donald Trump suggested he would slap Europe with tariffs after he hit America’s biggest trading partners — Canada,