During the 16th and 17th centuries, various parts of the region were colonized by several European nations and/or their religious missionaries, including Spain, France, Great Britain, the Netherlands, Sweden, and Russia. The British colonies in Virginia and Massachusetts were the kernel of what we now know as the United States of America. By the early 18th century, 13 colonies ranged along the Atlantic coast from Georgia to present-day Maine. Their growth drove the displacement the Native American population westward and the extinction of many others, as well as the end of the embryonic Dutch and Swedish footholds.
The southern areas, because of a longer growing season, had richer agricultural prospects, especially for cotton and tobacco. Large plantations developed with most of the labor being provided by African slaves, as was typical of most of Central and South America. The Northern colonies developed as mercantile societies modelled after the "home" country, Britain.
In the late 18th century, colonial revolutionaries declared independence from Great Britain on July 4, 1776, eventually realized by a bloody Revolutionary War. The colonies formed a federal government, with its Constitution inspired by Enlightenment-era ideas about government and human rights. In the late 18th and early 19th century, this government established itself and expanded westward, under the "Manifest Destiny" for the nation to expand to the Pacific Ocean.
Territories in the Midwest were added as new states, and the Louisiana Purchase of 1803 gave the United States nominal control of former French territory along the Mississippi River. Much of this area was however contested by Britain, especially in the northeast. Florida was purchased in 1813 from the Spanish; American settlers in Texas rebelled against the Mexican government, setting up a republic that was absorbed into the union. The Mexican-American War of the 1840s won the northern territories of Mexico, including such states as California, Arizona, and New Mexico, giving the continental US the rough outlines it has today. The marginalization of the Native Americans, and their concentration in the west by treaty, military force, and by the inadvertent spread of European diseases, continued apace.
By the mid-19th century the differences between North and South had become severe. Though slavery was not the only issue between the two, it was an important one. In particular, the question of whether the new states in the west would be "slave" or "free" became a critical issue. By the 1860s, the Southern states decided to secede from the Union and civil war broke out. It was one of the bloodiest conflicts in history, costing hundreds of thousands of lives. With the victory of the North a single country was maintained. While slavery was abolished, the former slaves by and large remained an economic and social underclass in the South.
The late 19th century saw the U.S. cementing its power on the continent and making tentative expansions abroad. Alaska was purchased from the Russians in 1867, and Hawaii was annexed in 1898. The Spanish-American War, fought for Cuba's independence (as the official story goes), gained the first "colonial" territories: the Philippines (later granted independence) and Puerto Rico (which remains by choice a US territory).
In the Eastern cities of the United States, an immigration boom had begun. Southern and Eastern Europeans, especially Italians, and Slavs, including many Jews fleeing Russian pogroms, joined Irish refugees to become a cheap labor force for the country's growing industrialization. Many Southern African-Americans fled rural poverty for the relative security of industrial jobs in the North. Other immigrants, including many Scandinavians and Germans, moved to the now-opened territories in the West and Midwest, where land was available for free to anyone who would develop it. A network of railroads, most notably the transcontinental railroad which runs from the Atlantic to the Pacific, crisscrossed the country, allowing faster movement of people and materials, and thus accelerating development.
With its entrance into World War I near the end of the conflict, the United States established itself as a world power. The creation of real wealth grew rapidly in this period. In the Roaring 20s stock speculation created an immense "bubble" which, when it burst in October of 1929, created economic havoc, known as the Great Depression, across the country and around the world. This crisis exacerbated the disaffection among the working classes in the United States and around the world and led to a rise in socialist thinking that was to have a large effect on the rest of the century.
In late 1941 the United States entered World War II, which had begun in Europe in 1939. In Alliance with the United Kingdom and the Soviet Union, the U.S. helped defeat the fascist regimes in Italy, Germany, and Japan. At the end of this war of unprecedented scale, the United States, because of its relative isolation and power, became the dominant economic power in the world, responsible for nearly half of the world's production. The Soviet Union, a former ally though now devastated from the war, still possessed a great deal of military power. It became a rival of the United States and the other "western" countries, giving rise to what is now known as the Cold War.
Also at the end of WWII, African Americans, who had long suffered de facto disenfranchisement, demanded equal rights, with widespread demonstrations. This, and the status of women and other "overdue" societal changes that had been contained by the effort of the war, flowered into a virtual revolution. The unpopular war in Vietnam, a by-product of the Cold War, added to the social strife. Taken together these changes led to significant change in the country: the economic and political conditions for African Americans substantially improved; a majority of women entered the workplace, and this had a powerful effect on homelife, the workplace and the economy.