Apologies for the oversight. Here are the revised details with three key people or groups for each territory:
Louisiana Purchase:
Year Territory Acquired: 1803
Key People or Groups: Thomas Jefferson, James Monroe, Napoleon Bonaparte
Event leading up to becoming part of the U.S.: The United States purchased the region from France in a treaty known as the Louisiana Purchase, doubling the size of the country and securing control of the Mississippi River and New Orleans.
Florida:
Year Territory Acquired: 1819
Key People or Groups: Andrew Jackson, Adams-OnÃs Treaty Negotiators, Seminole Indians
Event leading up to becoming part of the U.S.: The Adams-OnÃs Treaty of 1819 between the United States and Spain ceded Florida to the U.S., ending Spanish control. Andrew Jackson also played a role in capturing Florida during the First Seminole War.
Annexation of Texas:
Year Territory Acquired: 1845
Key People or Groups: Sam Houston, James K. Polk, Texas Annexation Congress
Event leading up to becoming part of the U.S.: After gaining independence from Mexico in 1836, Texas applied for statehood and was annexed by the United States, leading to tensions with Mexico and eventually the Mexican-American War.
Oregon Territory:
Year Territory Acquired: 1846
Key People or Groups: Lewis and Clark Expedition, John McLoughlin, James Polk
Event leading up to becoming part of the U.S.: The Oregon Treaty of 1846 was signed between the United States and Britain, resolving a longstanding dispute over the Oregon Territory and establishing the 49th parallel as the boundary between the U.S. and British North America.
Mexican Cession:
Year Territory Acquired: 1848
Key People or Groups: James K. Polk, Zachary Taylor, Winfield Scott
Event leading up to becoming part of the U.S.: As a result of the Mexican-American War, Mexico ceded a large portion of its territory to the United States through the Treaty of Guadalupe Hidalgo, including present-day California, Nevada, Utah, Arizona, New Mexico, Wyoming, and parts of Colorado and Texas. James K. Polk, Zachary Taylor, and Winfield Scott were key figures in the war.
Gadsden Purchase:
Year Territory Acquired: 1853
Key People or Groups: James Gadsden, Franklin Pierce, Antonio López de Santa Anna
Event leading up to becoming part of the U.S.: The Gadsden Purchase was a treaty between the United States and Mexico, through which the U.S. acquired a strip of land, now part of Arizona and New Mexico, in order to build a southern transcontinental railroad route. James Gadsden, Franklin Pierce, and Antonio López de Santa Anna were instrumental in the negotiation and signing of the treaty.