If electric power, internal combustion, and organic chemistry opened a new economic frontier at the turn of the twentieth century, the end of World War II signaled its impending close. Expansion continued during the postwar era, to be sure. Frontier ideology and frontier imagery continued to color people's thinking about the nature and future of American society. But there were signs at the end of the war that even the modern industrial world had finite limits, and the ensuing quarter-century seemed to bear them out.

The atomic bomb not only gave people the power to eradicate civilization as they knew it but, as radioactive fallout began making its way through the environment into commercial dairy products during the early 1960s, gave tangible proof of the ecological links among people, other living things, and the environment. Synthetic pesticides and antibiotics, likewise, promised at first to exterminate “enemy” insects and bacteria but, as those organisms grew resistant to the changing chemistry of their environment, once again taught people that nature was a living thing in which their lives were embedded and not merely the passive object of technological manipulation.