In 1968, Josef Koudelka was a 30-year-old acclaimed theater photographer who had never made pictures of a news event. That all changed on the night of August 21, when Warsaw Pact tanks invaded the city of Prague, ending the short-lived political liberalization in Czechoslovakia that came to be known as the Prague Spring. Koudelka had returned home the day before from photographing gypsies in Romania. In the midst of the turmoil of the Soviet-led invasion,...