1840s
- November 10, 1842
The Webster–Ashburton Treaty settled the border between the United States and lands held by the United Kingdom east of the Rocky Mountains, ending the disputes over the northern border of the state of Maine and northeastern border of Wisconsin Territory, which today resides in present day Minnesota.
- March 3, 1845
Florida Territory was admitted as the 27th state, Florida.
- December 29, 1845
The Republic of Texas was admitted as the 28th state, Texas. The United States Congress passed the joint resolution of annexation on March 1, 1845, but Texas did not agree to join the union for some time after. Although the annexation resolution avoided specifying Texas's boundaries, the U.S. inherited Texas's unenforced claims to South Texas, West Texas, over half of New Mexico, a third of Colorado, and small parts of Oklahoma, Kansas and Wyoming. With Texas joining the union, Arkansas finally gave up its claim on Miller County.
- June 18, 1846
The Oregon Treaty established the 49th parallel west of the Lake of the Woods as the continental border (so it did not include Vancouver Island) with the lands held by the United Kingdom. The sharing of Oregon Country ended, and the American portion becomes unorganized territory.
- August 15, 1846
U.S. Army of the West (1846) under Stephen Kearny captures New Mexico and claims it for the U.S. He installs a U.S. military government of New Mexico under the Kearny Code and later a provisional government of New Mexico, subject to the federal government and not to Texas.
- December 28, 1846
The southeast portion of Iowa Territory was admitted as the 29th state, Iowa. The remainder became unorganized.
- January 13, 1847
Californio resistance to U.S. forces in California ended with the Treaty of Cahuenga.
- March 13, 1847
The District of Columbia retroceded its land south of the Potomac River back to Virginia. Congress passed the retrocession act on July 9, 1846, and Virginia took possession of the land on this date.
- February 2, 1848
The Treaty of Guadalupe Hidalgo ended the Mexican-American War. Mexico ceded the Texas-claimed areas as well as a large area of land consisting of all of present-day California, Nevada, and Utah, most of Arizona, and portions of Colorado, New Mexico, and Wyoming.
- May 29, 1848
The southeastern portion of Wisconsin Territory was admitted as the 30th state, Wisconsin. The remainder became unorganized.
- August 14, 1848
Oregon Territory was organized, including present-day Idaho, northwestern Montana, Oregon, Washington, and western Wyoming.
- March 3, 1849
Minnesota Territory was organized, consisting of present-day Minnesota, and eastern portions of North Dakota and South Dakota.
Read more about this topic: Territorial Evolution Of The United States