20th Century
- June 14, 1900
The annexed Hawaiian islands were organized as the Territory of Hawaii, and corresponded, except for including Palmyra Atoll, to the present-day state of Hawaii.
- October 20, 1903
The Alaska boundary dispute is resolved by arbitration, generally favoring the American claim.
- November 16, 1907
Oklahoma Territory and Indian Territory were combined and admitted as the 46th state, Oklahoma. Present day states and future states now have current borders.
- January 6, 1912
New Mexico Territory was admitted as the 47th state, New Mexico.
- February 14, 1912
Arizona Territory was admitted as the 48th state, Arizona.
- August 24, 1912
The District of Alaska was organized as Alaska Territory.
- March 28, 1921
The dispute over a square mile between Delaware and Pennsylvania, The Wedge, is finally resolved in favor of Delaware.
- January 3, 1959
Alaska Territory was admitted as the 49th state, Alaska.
- August 21, 1959
Hawaii Territory was admitted as the 50th state, Hawaii, resulting in the present-day situation of the United States. The statehood act specifically excluded Palmyra Atoll from the new state; it thus became unorganized land. Since it had been incorporated as part of the Hawaii Territory, Palmyra Atoll became the only incorporated territory left in the United States.
- January 14, 1963
The Chamizal Dispute with Mexico over about 600 acres (2.4 km2) on the U.S.-Mexico border between El Paso, Texas, and Ciudad Juárez, Chihuahua is resolved. It was caused by differences between the bed of the Rio Grande as surveyed in 1852 and the channel of the river in 1895.
- 1970
The small town of Rio Rico, Texas, was ceded to Mexico in the Boundary Treaty of 1970. The handover officially took place in 1977, and the town was added to the Mexican state of Tamaulipas, of which it had long considered itself a part. See Mexican Boundary Exchanges.
Read more about this topic: Territorial Evolution Of The United States