The fog came down over Brooklyn at daybreak, so thick that, as David McCullough tells it, you could barely make out a man six yards away. Across the river in Manhattan, there was no fog at all. Under that cover, the last of Washington’s army slipped away from the British, who had them trapped against the East River. It was August 30, 1776. Had the sun come up clear, the war might have ended that morning.

McCullough told the story to the Providence College class of 2018 as proof of how much luck shapes history. Hard to argue. Nobody ordered that fog.