Isolationism
Ernie,
This is a very basic and readable outline of the history of American Isolationism. As I say, it's very basic but you'll get a good idea of why we abandoned our isolationist tendency until WWI and II. With the Cold War, the beginning of NATO, the UN, and global oil interests it became pretty difficult to stick to the policies of the 19th century.
So, we have been called the world's police. Mostly we get called that by countries that have benefited handsomely from our having military bases in those countries. During the Cold War no one seemed to object to those bases. We also share a lot of military technology with them, too, and they with us. Some countries aren't always happy to return the favor. When we bombed the North African country of Libya after an act of terrorism on their part, we were denied the quicker route over French air space by the French government. I watched a video recently where Libya's leader praised France for her decision.
The point is it's a terribly complex subject even though many look at it in a very simplistic way and I wish you all the best in your assignment.
ML