When the Wright brothers invented their flying machine, Americans lived in a nation of two dimensions, circumscribed by lines drawn on a conventional map. A century later, their nation existed in fact, reigned in three dimensions. Two million Americans 'slipped the surly bonds of earth' daily, carried aloft by aircraft operating in every part of the world. The airplane turned the sky into a new domain of human activity, a fast developing frontier first braved by adventurous young men. Then came the rich and the hurried, followed by just about everybody else. Until now, no one has told the story of aviation as one of frontier expansion. Aviation's frontier stage lasted a scant three decades, then vanished as flying became a settled experience. Sky as Frontier shows how commercial and military imperatives destroyed this pioneer world by routinizing flight. Along the way, Courtwright stops to consider dogfighting, barnstorming, the first air mail pilots, the development of airlines, air power during World War II, flight's impact on the environment, the troubled space frontier, and how the male dominated aviation enterprise was domesticated and democratized. The history of American flight shows a fateful trade off. Rationalization killed the adventure in flying but made possible rapid aerial expansion. With it came commercial growth and global military reach. In no other country did social life, business, and military operations become so intertwined with aerospace advances or have such large consequences for national power and prestige.