The United States acquired the land west of the Mississippi through several means, including treaties, purchases, and wars.
One of the earliest acquisitions was the Louisiana Purchase in 1803, which was a treaty between the United States and France for the vast territory west of the Mississippi River. It nearly doubled the size of the United States.
Next, the United States acquired the Oregon Territory from Britain in a treaty in 1846.
The Mexican-American War, fought from 1846 to 1848, resulted in the United States gaining a significant amount of territory, including California, Arizona, Nevada, and parts of Utah, Colorado, New Mexico, and Wyoming.
Finally, in 1853, the United States made the Gadsden Purchase, buying a strip of land along the southern border of present-day Arizona and New Mexico from Mexico.
Through these means, the United States gained nearly all of its current territory.