the US is a federal republic and a constitutional representative democracy
For many Americans, the American Revolution chained their views of the enslavement of African Americans in that they started to view slavery as a hypocritical evil since they had just fought for their own "freedom" from Great Britain.
Probably the kingdom of axum
It meant that as soon as one country falls under communist control, another country would fall and so on. Vietnam at the time was split into North (communist) and South (being attacked by North) and the US didn't want communism to spread, reasoning that it might eventually come to the US if nothing was done.
