In the spring of 1803, the western boundary of the United States had reached the Mississippi River, and the original thirteen states had expanded to seventeen with the admission of Vermont (1791), Kentucky (1792), Tennessee (1796), and Ohio (1803). By the end of the year, the size of the country had doubled with the Louisiana Purchase, bringing under American control land stretching from the Gulf of Mexico to British Canada and from the Mississippi River to the Rocky Mountains.
Then, in less than a half century, the map of the United States again changed dramatically through a combination of diplomacy and war. Florida was acquired from Spain in 1819. Nagging questions over the border with Canada were worked out through the Webster‐Ashburton Treaty
(1842), and the long‐standing dispute with Great Britain over the Oregon Country was resolved by establishing the forty‐ninth parallel as the boundary (1846). Texas, which won its independence from Mexico in 1836, was admitted to the Union in 1845. As a result of war with Mexico (1846–48), almost all of the Southwest, including the remainder of Texas, New Mexico, and California, was ceded to the United States.