Answer: Here's all the land pieces I know
Louisiana Purchase: Sold by Napoleon to the US in 1803
Flordia: The Adam-Onis Treaty seceded Florida the US
CA, NV, UT, AZ: Treaty of Guadalupe Hidalgo ended the Mexican-American war and these territories are called the Mexican cession.
Texas: (kinda complicated) Texas won its own independence and became the Lone Star state. Because of slave issues they did not admit Texas for a number of years. The Mexcian-American war did secure a lot of these lands
Oregon Territory: This land was previously co-ruled by Britsh and the US. They wanted this to end so they divided the land in half at the 49th parallel. President Polk tried to push for more land but was rejected.
Gadsden Purchase: Last purchase of land until Alaska or Hawaii. Bought from the Mexican government to build an easier railroad route (avoid tunning through the Rocky Mountains). This is a small portion of land below New Mexico.