The 1st great expansion of the country came with the Louisiana Purchase of 1803, which doubled the US territory, but brought it into minor conflict with the colonies of Spain which eventually resulted in the acquisition of Spanish Florida.
Webster-Ashburton Treaty of 1842 with Britain split the disputed territory in Maine and New Brunswick and finalized the border with Canada, including the disputed Indian Stream territory.
The annexation of the Republic of Texas in 1845 led directly to the Mexican–American War, after which the victorious United States obtained the northern half of Mexico’s territory (1846), including what was quickly made the state of California.
Texas Annexation was annexed in 1845, New Mexico was captured by the U.S. Army in 1846.
Oregon politicians hoped to continue Oregon’s political evolution into an independent nation, but the pressure to join the U.S. would prevail by 1848, four months after the Mexican–American War.
U.S. Population changes