Give examples of how the United States has been a force for good and evil in the 20th century.
This is, of course, to some extent a matter of opinion. Good and evil are to some extent in the mind of the beholder.
I would argue that the clearest example of the US as a force for good might be World War II. The Nazis were one of the clearest examples of evil in the history of the world and the US helped to defeat them. The Japanese empire, at that time, was also willing to commit atrocities like the Unit 731 horrors in China.
It is harder to argue that the US has been a force for evil. Perhaps the best argument has to do with US support for nasty regimes during the Cold War. The best example of this was the apartheid regime in South Africa which the US supported regardless of its record on human rights.