It was soon clear, since the nineteenth century, that the United States did not show up as an area in need of back and go through various stages of development before reaching the terminus of the complex European civilization, but constituted an original and indigenous way for economic and social development. The existence of a moving frontier to the west and a vast virgin territory to conquer, open to the adventurous spirit of the pioneers and settlers, an extraordinary chance in the history of the nineteenth century. Across the middle of the century, in fact, the states members of the Union were now over thirty and occupied a boundless territory that begins from the Atlantic Ocean to the Great Lakes and down the valley of the Mississippi to reach from one side of the Gulf Mexico until the Pacific Ocean. The independence of Texas before (1836) and the war with Mexico later (1846-48) had in fact converted into U.S. territory even the Southwest. The consequence of this phenomenon was that the development of the West and South could go hand in hand, without excessive imbalances, even considering the rapid development of industry in the Northeast. The rural production could in fact meet with the amount of land available to the backwardness of the techniques used by the settlers.
On the economic and social point of view, however, the United States found themselves divided into three quite distinct areas: the North with its dynamism of industrial labor, the South with large plantations and slave labor, the West characterized by farmers and ranchers. The conflict did not wait, even if it was temporarily dammed by the Missouri Compromise of 1820 which established, the parallel 36° 30', the dividing line between slave states and non-slave states. With the arrival of Lincoln to the presidency, in 1860, the first latent conflict started and the secession of the Confederate States of the South was followed by, starting from 1861, a very bloody civil war that saw, for the first time, full employment, for military purposes, of large-scale industry and in particular the railways, used for the transport of troops and heavy weapons. After an initial phase in favor of the Confederate armies, the North, with its strong economic potential, took over in 1865 and came out victorious, and the U.S., with the abolition of slavery and the emancipation of blacks, could be structurally unified. So it was, in a way, completed the second stage of the American Revolution.
The reconstruction, carried out in the South in a difficult social climate, nevertheless stimulated a great momentum and industrial production. At the end of the century, the United States could boast, and rightly so, to be entered among the great powers. It revealed, among other things, as a huge ethnic and racial "melting pot" able to absorb massive immigration of labor from Europe (and later from Asia ), giving newcomers work and living opportunities. After the arrival of the British, Irish and Germans in the second half of the nineteenth century and the beginning of the next century it was the turn of the peoples of the Hapsburg Empire, Scandinavians, Italians, Jews, Poles, Russians, and on the shores of the Pacific, Asians. We can calculate that almost twenty million people reached the United States between 1870 and 1920.