Most of the US Imperial territory was taken from the Spanish in the Spanish-American War in 1898. Some other territories came as a result of negotiations with the other Great Powers and some more territories came as a result of World War II.
In 1898 by Act of Congress.
In 1898 as a Territory by Act of Congress.
Alaska was a US Territory long before the Spanish American War.
63 years
Louisiana was a French territory purchased by United States in 1803; Louisiana was not a Spanish territory.
France
They purchased it from France.
ALASKA.
The US acquired the territory of Florida from Spain ceding it to the US.
In 1846 Britain agreed for the US to take control of the territory
In 1898 by Act of Congress.
Florida
In 1898 as a Territory by Act of Congress.
American Samoa.
He did indeed- paid 15 million dollars for this territory in 1803.
They still hold Guam and Puerto Rico as US Territories.
Louisiana Territory was purchased in 1803. Florida Territory was purchased in 1821, California, New Mexico and Arizona Territory followed with the Mexican American War. Alaska in 1867.