In the United States, a complication is that our real folklore isn't called "folklore."
When you say "folklore" I tend to think of stories, often ethnic stories, like Brer Rabbit and the tar-baby, or John Henry, the steel-drivin' man, or Paul Bunyan. In reality I don't think these are a living part of our culture any more.
Our real folklore is probably what we see in movies... or comic books, excuse me, graphic novels, like the Marvel "universe."
There are exceptions. Certainly I learned these pieces of folklore. I won't try to explain how wrong they are. These descriptions are exaggerated--but not by much. Most Americans today know they are not true. (I hope). Yes, they might be 10% true.
1) Columbus knew the world was round, when everybody else thought it was flat. His sailors were afraid they would sail off the edge of the world, but he knew better, and he kept going. Columbus discovered America (i.e. the land that is now the US). He was a great and admirable hero in every way.
2) "O beautiful for pilgrim feet
Whose stern, impassioned stress
A thoroughfare for freedom beat
Across the wilderness!"
--Second verse of "America, the Beautiful," by Katharine Lee Bates
The Pilgrim Fathers sailed from England to what is now Plymouth, Massachusetts in search of religious freedom. At that time, North America was a completely empty expanse of trackless wilderness. The United States expanded into this unused, empty land. Instead of fighting wars like Europeans, we bought our land, legally, from France and other countries. There were a few Indians living here and there, but they weren't really entitled to the land because they weren't really using most of it.
3) Everybody always knew Hitler was evil. Nobody in the United States ever had anything good to say about Hitler or the Third Reich. We fought Hitler because he was killing Jews in extermination camps.