Paris in the '70s was running at full tilt -- a new order was coming into view amid hedonism, ambition, and outrageous decadence. This was a fashion revolution, the beginning of fashion as rock-star spectacle, and the world went wild for it. Alicia Drake writes about the dramatic collision and rivalry between two titanic geniuses, Yves Saint Laurent and Karl Lagerfeld, whose clashes sparked that tumultuous decade. Americans came to Paris in droves...