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.
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The Soviet Union set off an atomic bomb
It set the border between the U.S. and Canada at 49 degree North Latitude as far west as the rocky mountains. And Us and Mexico
What caused a fear of communism, and how did it change American policy
the u.s. purchased florida territory from spain and seven states were admited into the unioun