President Trump is threatening 25% tariffs on Japan and South Korea after months of trade talks have yet to seal a deal. The S&P 500 turned lower in Monday afternoon trading, but investors know there's a good chance this won't be the final word.

President Trump posted letters to Japanese and South Korean leaders on social media, informing them he plans to impose 25% tariffs on their goods. He has said more letters to leaders will be coming.

The Trump administration is expected to send out the first batch of letters announcing new tariffs on countries at midday Monday, after delaying a self-imposed deadline to make deals with dozens of trading partners.