Why did the United States come into consideration in any study of Latin American civilization?

Answer 1

In the 1800's, many nations in Latin America were fighting for independence. Under a policy referred to as the Monroe Doctrine, the United Stated proclaimed that any nation which attacked these fledgling nations would be considered to be attacking the United States, also. Essentially, we tried to act as big brothers--I can beat up my little brother, but you can't.

Answer 2

The US is sort of the elephant in the room -- too big to ignore. Most anything that the US does or did would have its effects in Latin America.