John Tierney, Nicole Gelinas, Park MacDougald, and Isabella Redjai pay tribute to Charlie Kirk, assassinated as he gave a talk on a college campus. Kirk was murdered the day before the nation marked the 24th anniversary of the 9/11 attacks. We share personal memories of that day, examine the city's response, and discuss how the attacks reshaped American politics and culture. The conversation ends with a look at the day's enduring lessons and prospects for American unity.