President-elect Trump’s pledge to impose tariffs on Mexico and Canada threatens to upend decades of North American integration, a bumpy process that’s recast the two U.S. neighbors as providers of raw and finished goods for the world’s biggest economy.

Trump, who takes office on Jan. 20, said he would impose a 25% tariff on imports from Canada and Mexico until they clamped down on drugs, particularly fentanyl, and migrants crossing the border, in a move that would appear to violate a free-trade deal.

Mexico and Canada are the United States’ top two trading partners, accounting for nearly 30% of trade volumes.