As history shows, the concept of America as an expansive global empire can be an intoxicating political force.

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."