President Donald Trump cranked up the pressure on two of America’s key trading partners, Japan and South Korea, firing off letters Monday to the heads of their countries that informed them of their new tariff rate.

Trump initially sparked hysteria in the financial markets by announcing tariff rates on dozens of countries, including 24% on Japan and 25% on South Korea. In order to calm the markets, Trump unveiled a 90-day negotiating period during which goods from most countries were taxed at a baseline 10%.