September 11 is the obvious top contender. But 2016 was the first election featuring voters with no memory of that day—which will only be increasingly common in 2020 and beyond. As some commentators were already writing a few years ago, “the 9/11 era is over.”

In no way do I mean to minimize the tragedy of September 11. The tragic 1915 sinking of the Lusitania by Germany caused 1,198 deaths and helped galvanize the American public into World War I, just as 9/11 led to our military interventions in Afghanistan and (much more controversially) Iraq. But the Lusitania is not widely remembered today by the general public. Perhaps it would have fared better if James Cameron had made a movie about star-crossed lovers on that ship instead.

Another applicable analogy here might be the Kennedy assassination. But if we assume a minimum age of five to form a clear memory, only about 21 percent of Americans still remember the Kennedy assassination from their own experience—and that was just 55 years ago.