The postwar years were a true flowering of American culture. Even as the United States was locked in an arms race with the Soviet Union, which culminated in the terrifying doctrine known as mutually assured destruction, the country evolved from a military and economic powerhouse into a cultural presence at the center of the world. Modern jazz and rock and roll were exported and celebrated around the globe. Painters came out of the long shadow of war-torn Europe and led the way into new forms of abstraction and social commentary. Thinkers like James Baldwin turned a spotlight back on America’s fundamental, unexamined flaws. This period, in all its complicated glory, is the subject of “The Free World: Art and Thought in the Cold War,” by Louis Menand. Menand is a professor at Harvard University and a winner of the Pulitzer Prize for his book “The Metaphysical Club,” from 2001. Menand talks with David Remnick about a time, as he writes, when “ideas mattered. Painting mattered. Movies mattered. Poetry mattered.”