The U.S. Senate (the upper house of Congress) approves treaties between the United States and other countries. However, boundary changes of states that lie on international borders require the approval of the state legislature in that state. Boundary changes between states require the approval of each state's legislature and the approval of Congress. The U.S. Supreme Court settles boundary disputes between states.
1782-1783: Treaties with the United Kingdom establish the U.S. as an independent country and establish the boundary of the United States as being bound on the north by Canada, on the south by Spanish Florida, on the west by the Mississippi River, and on the east by the Atlantic Ocean.
1803: The Louisiana Purchase extends the western boundary of the United States to the Rocky Mountains, occupying the drainage area of the Mississippi River, as estimated by the French explorer Robert La Salle. The Purchase doubled the territory of the United States.
1818: A convention with the United Kingdom established the northern boundary of the Louisiana Purchase at 49 degrees north.
1819: Florida was ceded to the United States and purchased from Spain.
1820: Maine became a state, carved out of the state of Massachusetts. The northern boundary of Maine was disputed between the U.S. and Canada so the King of the Netherlands was brought in as an arbiter and he settled the dispute in 1829. However, Maine refused the deal and since Congress requires the approval of a state legislature for boundary changes, the Senate could not approve a treaty over the border. Ultimately, in 1842 a treaty established the Maine-Canada border of today although it provided Maine with less territory than the King's plan would have.
1845: The independent Republic of Texas (1836-1845) is annexed to the United States. The territory of Texas extended north to 42 degrees north (into modern Wyoming) due to a secret treaty between Mexico and Texas.
1846: Oregon Territory is ceded to the U.S. from Britain following an 1818 joint claim on the territory (which resulted in the phrase "Fifty-Four Forty or Fight!"). The Treaty of Oregon establishes the boundary at 49 degrees north.
1848: The Treaty of Guadalupe Hidalgo following the Mexican War between the U.S. and Mexico resulted in the purchase of Arizona, California, Nevada, New Mexico, Texas, Utah, and western Colorado.
1853: With the Gadsden Purchase of 1853, the land acquisition that resulted in the area of the 48 contiguous states today was completed. Southern Arizona and southern New Mexico were purchased for $10 million and named for the U.S. minister to Mexico, James Gadsden.
1862-1863: When Virginia decided to secede from the Union at the start of the Civil War (1861-1865), the western counties of Virginia voted against the secession and decided to form their own state. West Virginia was established with help from Congress, who approved of the new state on December 31, 1862 and West Virginia was admitted to the Union on June 19, 1863. West Virginia was originally going to be called Kanawha.
1867: Alaska was purchased from Russia for $7.2 million in gold. Some thought the idea was ridiculous and the purchase became known as Seward's Folly, after Secretary of State William Henry Seward. The boundary between Russia and Canada was established by treaty in 1825.
1898: Hawaii was annexed into the United States.
1925: The final treaty with the United Kingdom clarifies the boundary through the Lake of the Woods (Minnesota), resulting in the transfer of a few acres between the two countries.