Andrew Johnson was Vice President of the United States under President Abraham Lincoln. Following Lincoln's assassination, Johnson became the 17th President of the United States.
In the Timeless Universe
During the original events of Abraham Lincoln's assassination, Johnson was also targeted for assassination but escaped unharmed. His intended assassin, German-immigrant George Atzerodt, lost his nerve, got drunk and left.
At a hotel on April 14th, 1865, George Atzerodt gathers his courage and takes a shot at Johnson with a modern handgun provided by Garcia Flynn. Atzerodt is tackled by Rufus Carlin, causing him to miss and wound a Union soldier, saving Johnson's life. This wounded Union soldier is given credit for saving Johnson, Rufus's role being forgotten to time.
Andrew Johnson at Wikipedia