What caused the change in the US boundaries?

There are several reasons for the changes. The US purchased land from France known as the Louisiana Purchase and then they had an expedition by two people known as Merriweather Lewis and William Clark explored land along the Mississippi, Missouri, and Snake River and claimed any of the land that wasn't already the US's the US's (this included Oregon). Texas asked to join the US but Mexico didn't want to give it up so America a few wars with Mexico and got more land each time. Mexico also gave California to the US and the US bought some land from Mexico as well and Alaska and Hawaii voted to join the United States as well.