President Trump announced Thursday his administration would impose a 35% tariff on all Canadian goods beginning next month.

Trump posted a letter sent to Canadian Prime Minister Mark Carney in which he outlined the upcoming tariffs. This week, Trump has posted letters to more than a dozen countries vowing to impose steep tariffs on their imports starting Aug. 1.

The president argued Canada had not done enough to curb the flow of fentanyl into the United States, even though relatively little fentanyl crosses the northern border each year compared with the southern border.

“If Canada works with me to stop the flow of Fentanyl, we will, perhaps, consider an adjustment to this letter,” Trump wrote to Carney. “These Tariffs may be modified, upward or downward, depending on our relationship with your Country.”

The U.S. had previously imposed a 25% tariff on Canadian goods, though Trump later exempted products covered under the 2020 trade agreement struck between the U.S., Canada and Mexico. It’s unclear whether those exemptions will still apply as of Aug. 1.

Trump has for months railed against Canada, claiming the U.S. has no need for Canadian goods and musing about annexing Canada as the 51st state. 

Carney and other Canadian leaders have flatly rejected that suggestion and asserted that the two nations work better together as partners.