Heather Swan is a writer with a gift for listening to the natural world. Still, she didn't know what to make of the barred owl who came to visit her every day for three weeks. And then she realized, with a jolt, the owl had a message for her.

Why do humans still wage wars? Despite their terrible costs, they benefit certain groups, and thoughout history, they've also galvanized social movements and sparked scientific advancements. Margaret MacMillan explains how wars have shaped us.