Donald Trump says the U.S. needs control of Greenland and the Panama Canal "for national security." Here's why both places are so important.

Authorities of Panama and Greenland have repeatedly said they won't hand over any territory to Trump, with Panama's president declaring that the canal "is going to stay in our hands forever," and Denmark's prime minister insisting "Greenland is not for sale."

President-elect Donald Trump wouldn't rule out military or economic coercion, saying the U.S. needs to stop China. Also a factor: climate change.