Don't Mess With Earth

An advanced group of humans leave ancient Earth, but keep an eye on their less advanced cousins. The advanced humans meet the Ragnor, an alien species that experiments on other species for no other reason but that they can.  The advanced humans begin to manipulate Earth history by posing as historical figures such as King Arthur, Galileo, Amelia Earhart, among others. The Ragnor find Earth, causing all sorts of historical anomalies such as the Salem Witch Trials and the disappearance of the Roanoke colonists. In the 1940's, one of their ships get shot down over New Mexico, by some American fighter pilots with some amazing flying and some lucky machine gunning. Once the scientists of Earth start reverse engineering the downed spacecraft, President Truman issues an executive order that will forever change Earth history.