<P> The United States of America was created on July 4, 1776, with the declaration of independence of thirteen British colonies . Their independence was recognized by Great Britain with the Treaty of Paris of 1783, following the American Revolutionary War . This effectively doubled the size of the colonies, now able to stretch west past the Proclamation Line to the Mississippi River . This land was organized into territories and then states, though there remained some conflict with the sea - to - sea grants claimed by some of the original colonies . In time, these grants were ceded to the federal government . </P> <P> The first great expansion of the country came with the Louisiana Purchase of 1803, which doubled the country's territory, but brought it into minor conflict with the colonies of Spain which eventually resulted in the acquisition of Spanish Florida . The Oregon Country gave the United States access to the Pacific Ocean, though it was shared for a time with the United Kingdom . The annexation of the Republic of Texas in 1845 led directly to the Mexican - American War, after which the victorious United States obtained Mexico's northwestern quarter, including what was quickly made the state of California . However, as the development of the country moved west, the question of slavery became too much to ignore, as there was a struggle to keep the number of northern abolitionist states equal to the number of southern slave states, with vigorous debate over whether the new territories would allow slavery and events such as the Missouri Compromise and Bleeding Kansas . This came to a head in 1860 and 1861, when the governments of the southern states proclaimed their secession from the country and formed the Confederate States of America . The American Civil War led to the defeat the Confederacy in 1865 and the eventual readmission of the states to the United States Congress . </P> <P> The country's expansion beyond North America began in 1856 with the passage of the Guano Islands Act, causing many small and uninhabited, but economically important, islands in the Pacific Ocean and Caribbean Sea to be claimed . Most of these claims were eventually abandoned due to competing claims from other countries or the guano having been mined out . The Pacific expansion culminated in the overthrow of the Kingdom of Hawaii in 1893 and its annexation in 1898 . Alaska, the last major acquisition in North America, was purchased from Russia in 1867 . </P>

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