1. The US acquired many territories during this period. I will list them pretty much chronologically:
- In 1845, the US annexed Texas, when the Republic of Texas was incorporated in the larger United States.
- In 1848 with, after the war with Mexico, the United States acquired California, Nevada, Utah and Arizona.
- Gadsden Purchase of 1853 gave the US southwestern parts of what is now New Mexico and southern Arizona.
- In 1867, US bought Alaska
-In 1903, the Panama Canal Zone became a part of the US after acquisition by Theodore Roosevelt.
- After that multiple island nations become the part of the US such as the Philippines, Puerto Rico and Cuba.
2. When it comes to US expansion the most important thing to know is the Manifest Destiny and the New Manifest Destiny beliefs. They both state that the settlers are destined and have responsibility to settle and lead by example while also helping the less fortunete, first in the original Manifest Destiny, North America, and in new Manifest Destiny overseas territories. On the other there were those who opposed the oversees expansion especially of the United States. They stated that it is against the ideals of the United States whose main ideal is Freedom, while also costing money. The opponents that his expansion is hypocritical as it subjugated other countries for the benefit of the United States.
3. This one is a bit hard but I would say that truth is somewhere in the middle. There are countries that could befit from the US help and that could improve their lives, and the US can be a force for good. That is a noble goal. Unfortunately, that goal, in practice failed a lot of times, as greed and corruption and lies have subverted that goal and made it into what the opponents of the expansion said it was.