The main reason why the United States largely shied away from imperialism prior to 1890 was because it was still a relatively "fragile" nation with a host of its own issues to work out--such as the slavery issue that eventually led to the devastating Civil War.
I believe you're looking for manorialism.<span />
It's a because during the middle ages kingdoms conquer other nations by the numbers as soon as the child has education he (I say he because they didn't allow women to have education back then) he will enlist into one of the European nations and fight.
The federal government was the George Washington in convention
Terrorism, as the name implies, is the act of trying to inflict terror on the opposition, usually for change. It relies on striking fear into the enemy. Traditional warfare does not rely on fear and it's goals are to destroy military targets and other things that are useful o the enemy (in total war).
(hope I helped)
