Territorial Evolution of The United States - 1820s

1820s

March 16, 1820

The Maine District of Massachusetts was split off and admitted as the 23rd state, Maine, as part of the Missouri Compromise. The northern border of Maine was not settled until 1842.

July 10, 1821

The Adams-Onís Treaty or Transcontinental Treaty came into effect, establishing a defined border between the United States and New Spain. The treaty ceded Spain's claims to Oregon Country to the United States and American claims to Texas to Spain; moved portions of present-day Colorado, Oklahoma, and Wyoming, and all of New Mexico and Texas, to New Spain; and all of Spanish Florida to the United States. The new borders intruded on Arkansaw Territory's Miller County, created on April 1, 1820, which dipped below the Red River and into land now ceded to Spain. However, the remoteness of the region caused no serious conflict with Spain.

August 10, 1821

The southeastern corner of Missouri Territory was admitted as the 24th state, Missouri. The remainder became unorganized. Missouri did not include its northwestern triangle at this point, that being added later in the Platte Purchase.

September 16, 1821

Ukase of 1821 attempts to forbid non-Russian ships from approaching the Northwest Coast. Only attempt at enforcement is seizure of the U.S. brig Pearl in 1822. U.S. reacts with the Monroe Doctrine in 1823.

September 27, 1821

The Viceroyalty of New Spain achieved independence as Mexico. Spanish Texas became Mexican Texas.

March 30, 1822

East Florida and the portion of West Florida not already part of other states were combined and organized as Florida Territory, which corresponded to present-day Florida. Around this time, the official spelling of Arkansaw Territory became Arkansas Territory.

November 15, 1824

Arkansas Territory was shrunk, the western portion becoming unorganized.

January 12, 1825

Russo-American Treaty of 1824 gave Russian claims south of parallel 54°40′ north to the United States. This and the Anglo-Russian Convention of 1825 leave Britain and the U.S. as the only claimants to Oregon Country. Russia retained Fort Ross, California until 1841.

May 6, 1828

Arkansas Territory was shrunk further, attaining the present-day borders of Arkansas, with the remainder again becoming unorganized, excepting the land it still claimed as Miller County.