How has the United States acquired new territories?

Since the United States Constitution’s ratification in 1789, the country has grown from 864,746 square miles to 3,531,905 through territorial acquisitions.
According to the U.S. Department of the Interior’s Office of Insular Affairs, the United States has employed three methods of territorial acquisition:
  • Cession refers to a transfer of land that is formally agreed upon by the acquiring and ceding state, usually by treaty.
  • purchase is a type of cession in which the acquiring nation agrees to financially compensate the ceding country for a territorial transfer.
  • Occupation refers to the appropriation of an area that lacks supreme control by another sovereign.
The U.S. also leased the Panama Canal Zone from 1903 to 1999.
The country’s first and largest territorial acquisition was the Louisiana Purchase in 1803 for $10 million; it nearly doubled the landmass of the original 13 states. In 1947, the Mariana Islands, Caroline Islands, and Marshall Islands became the United States’ most recent territorial acquisitions.
The cheapest acquisition by purchase was of Alaska for $12 per square mile. The most expensive acquisition by purchase was of the U.S. Virgin Islands for $183,824 per square mile.
The territories gained by the U.S. through occupation were primarily small islands in the Pacific Ocean and the Caribbean Sea. The Guano Islands Act of 1856, which was designed to assist American farmers by making guano (dried sea bird excrement) easier to mine for use as fertilizer, authorized such occupations.
The United States currently controls five unincorporated, organized, inhabited territories: Puerto Rico, Guam, Northern Mariana Islands, the U.S. Virgin Islands, and American Samoa. The U.S. also controls nine minor outlying islands: Baker Island, Howland Island, Jarvis Island, Navassa Island, Johnston Atoll, Midway Islands, Palmyra Atoll, Wake Island, and Kingman Reef.