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.

But Trump’s concerns are also heavily influenced by something he often does not acknowledge: climate change and its growing influence on national security.Start the day smarter. Get all the news you need in your inbox each morning.