Any benefits, however, always were mixed with consequences that were far less happy and sometimes were grimly tragic. European American pioneers brought with them many diseases for which native peoples had little natural resistance. This biological frontier - epidemics of smallpox, cholera, influenza, measles, whooping cough, and other maladies - brought a calamitous decline in the American Indian population. By some estimates the number of native peoples in the Western Hemisphere may have dropped by 95 percent between 1500 and 1900, although other authorities place the figure at 50-60 percent.

Development of resources also triggered profound environmental changes, some of them with disastrous long-term results. From the Gulf coastal cotton frontier to the Great Plains and beyond, overcropping led to severe erosion, the silting of streams, and economic disasters such as the famous Dust Bowl. Mining regions also suffered from severe erosion after hills were stripped of trees used for buildings and mine timbers, while mining and smelting itself left the mountainsides scarred and the water often polluted. The frontier's wealth came at a considerable ecological cost.

Many of those on the frontier complained, furthermore, that most of that wealth was leaving for somewhere else. Outside centers of power, eastern and European cities sending corporate investments westward, reaped much of the profit from the mines, ranches, and lumbering operations. Westerners also protested that they remained politically underrepresented. The frontier, whether the Atlantic colonies in the 1700s, trans-Appalachia in the 1800s, or the Far West in the 1880s, was a kind of colony. Many areas of the West and South still protest that they are exploited by faraway power and capital.

The first pioneers, whose descendants later would complain of exploitation, had seized control of the West from dozens of Native American societies and from Hispanic and Franco-American peoples carried there by their own frontiers. With that military and political domination had come economic exploitation and the erosion of cultural life among some of the oldest and most deeply rooted societies on the continent. For all its contributions, westward expansion brought with it enormous loss and collective pain.