See the Louisiana Purchase, from Wikipedia:
The Louisiana Purchase (1803) extended United States sovereignty across the Mississippi River, nearly doubling the nominal size of the country. The purchase included land from fifteen present U.S. states and two Canadian provinces, including the entirety of Arkansas, Missouri, Iowa, Oklahoma, Kansas, and Nebraska; large portions of North Dakota and South Dakota; the area of Montana, Wyoming, and Colorado east of the Continental Divide; the portion of Minnesota west of the Mississippi River; the northeastern section of New Mexico; northern portions of