A day after levying steep new tariffs on Mexico, Canada and China, President Trump acknowledged what economists, members of Congress and even some of his own aides — in their previous lives — have been saying all along: Americans may find themselves paying the costs.

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.