Far more impressive was the almost miraculous expansion which came to the United States during this period. When the United States started upon their career as an independent nation in 1782, their territory was limited to the lands east of the Mississippi, excluding Florida, which was still retained by Spain. Only the eastern margin of this area was at all fully settled; and the population numbered at most 2,000,000, predominantly of British blood. In 1803, by a treaty with Napoleon, the French colony of Louisiana, with vast and ill-defined claims to the territory west of the Mississippi, was purchased from France. Meanwhile the stream of immigrants from the eastern states, and in a less degree from Europe, was pouring over the Alleghany Mountains and occupying the great central plain; and by 1815 the population had risen to almost 9,000,000, still mainly of British stock, though it also included substantial French and German elements, as well as large numbers of negro slaves. In 1819 Florida was acquired by purchase from Spain. In 1845-48 a revolution in Texas (then part of Mexico), followed by two Mexican wars, led to the annexation of a vast area extending from the Gulf of Mexico to the Pacific coast, including the paradise of California; while treaties with Britain in 1818 and 1846 determined the northern boundary of the States, and secured their control over the regions of Washington and Oregon. Thus the imperialist spirit was working as irresistibly in the democratic communities of the New World as in the monarchies of Europe. Not content with the possession of vast and almost unpeopled areas, they had spread their dominion from ocean to ocean, and built up an empire less extensive indeed than that of Russia, but even more compact, far richer in resources, and far better suited to be the home of a highly civilised people. Into this enormous area there began to pour a mighty flood of immigration from Europe, as soon as the Napoleonic wars were over. By 1878 the population of the States had risen to about 50,000,000, and was greater than that of any European state save Russia. A new world-state of the first rank had arisen. It was made up of contributions from all the European peoples. Those of British stock, especially the Irish, still predominated throughout this period, but the Germans and the Scandinavians were becoming increasingly numerous, and the Italians, Greeks, Poles, Czechs, Russian Jews, and other stocks were beginning to form very substantial elements. It was a melting-pot of races, which had to be somehow welded into a nation by the moulding-power of the traditions implanted by the earlier British settlers. It may fairly be said that no community has ever had imposed upon it a more difficult task than the task imposed by Fate upon the American people of creating a national unity out of this heterogeneous material. The great experiment was, during this period, singularly successful.