President Donald Trump late Thursday threatened a 35% tariff on goods imported from Canada, a dramatic escalation in an on-again, off-again trade war with America’s northern neighbor and one of its most important trading partners.

President Donald Trump threatened a 35% tariff on some Canadian goods and raised the prospect of increasing levies on most other countries, ramping up his trade rhetoric in comments that weighed on stocks and boosted the US dollar.

The president has published just 22 letters, many to minor trading partners like Brunei, Moldova and Sri Lanka.