President Donald Trump said the US will put a 35% tariff on some imports from Canada, escalating the tensions between two countries that have impaired one of the world’s largest trading relationships.

Trump has justified tariffs on goods from Canada, the top U.S. export market, by accusing it of failing to stop the cross-border flow of fentanyl, even though U.S. government data shows Canada accounts for only 1% of the fentanyl that enters the United States.

The imbalance in President Trump’s treatment of America’s closest trading partners may come from his desire to make Canada the 51st state, some Canadians believe.