Determining the most influential pop-culture figure of the 20th century is probably a fool’s errand. But if you truly had to choose one, your best bet would be Louis Armstrong. He was one of the forefathers of modern jazz, shaping that art form as we know it today. The story of America, one of both resplendent hopefulness and inhuman oppression, was written in his bones—he turned the Star-Spangled Banner into a fractured anthem of pride and frustration long before Jimi Hendrix did. And to hear and see him—singing in his incomparably expressive purr, hitting one of his famously sweet high C’s on the trumpet, dishing the dirt with amiable savoir faire on a TV talk show—was to love him. Through a career spanning more than half a century, the world came to ador…