In 1492, Christopher Columbus and his crew of ragtag, starving, near-mutinous sailors washed ashore in the Bahamas, "discovering" the New World and claiming ownership of it for the Spanish monarchy. The Taino Indians Columbus encountered—whose homeland he claimed for Spain—must have thought he was mad, suffering delusions of grandeur.
But, as we know, Columbus's arrival was indeed the first act in a centuries-long drama of colonization and conquest in which Europeans and their descendents largely displaced the Taino and their fellow Indians while remaking the Western Hemisphere in their own image.
How and why were the European colonists able to achieve such total dominance in far-off continents? Did the Europeans' power lie in their technological superiority, especially in weapons of war? Or was the European advantage ideological, rooted in the aggressive expansionism of crusading Christianity or the profit motive of entrepreneurial conquistadors? Was it simply a matter of the Europeans proving more brutally committed to a genocidal fight to the finish?
While a case can be made for the significance of any of these factors—or all of them—in truth the single most important factor in facilitating the European conquest of the Americas may be found, surprisingly, in a realm beyond simple human control: ecology.
Columbus's ships, and those of the innumerable Europeans who followed him to America, short-circuited millions of years of divergent evolution in the two hemispheres by rapidly introducing Old World plants, animals, and micro-organisms into New World environments, and vice versa. This manmade reunion of the ecologies of the hemispheres—dubbed "The Columbian Exchange" by historian Alfred Crosby—had dramatically asymmetric consequences for the peoples of the Old World and the New.
The New World happened to be much a healthier place than the Old before 1492, hosting few or none of the devastating diseases that continuously plagued the populations of Europe, Africa, and Asia. Thus, when Europeans arrived, they generally found life in the Americas to be at least as healthy as back home. By contrast, American Indians—never before exposed to vicious Old World pathogens like smallpox and thus lacking any immunities to them—began dying at apocalyptic rates. Many historians now believe that new diseases introduced after Columbus's arrival killed off as much as 90% or more of the indigenous population of the Americas.
The Indians' "Great Dying"—which may have killed as many as one out of every five humans alive worldwide in the sixteenth century—ravaged not only Indian bodies but entire Indian societies and cultures. The traumatized survivors were often left unable to mount any effective resistance against the incursions of the European colonists.
The Columbian Exchange became even more unbalanced with Europe's successful appropriation of New World staple crops originally developed by Indians. The adoption of efficient, carbohydrate-rich American crops such as corn, potatoes, and cassava allowed Europeans and Africans to overcome chronic food shortages. Thus, even while Native American populations were decimated by Old World diseases, European and African populations swelled as American crops helped to overcome Old World famine.
Simple demographic numbers tell the story of the Columbian Exchange most starkly. When Columbus sailed the ocean blue, Europe's population stood at about 60 million. Most historians now believe that the population of the Americas at the same time stood somewhere between 40 and 100 million. In other words, it is not just possible but quite likely that American Indians outnumbered Europeans outright. (At its peak just before the Spanish arrival, the Aztec capital city of Tenochtitlan was more populous, cleaner, and more beautiful than Paris.)
But by 1800, after three centuries of the Columbian Exchange, Europe's population had surged to 150 million, while that of the Americas' fell to 25 million—of which the vast majority were descendents of European colonists or African slaves, not American Indians.
The success of European imperialism in the Americas was underwritten by the ecological imperialism of the Columbian Exchange. The European colonists who would eventually found the settlements that would become the United States had a powerful—if accidental—ally in the environment itself.