The First Assassin was inspired by actual history–threats against Abraham Lincoln, real or imagined. The writings of Col. Charles P. Stone were a major source for the novel. Here’s what Stone wrote about the events that took place exactly 150 years ago this evening, the night before Lincoln’s first inauguration:
During the night of the 3d of March, notice was brought me that an attempt would be made to blow up the platform on which the President would stand to take the oath of office. I immediately placed men under the steps, and at daybreak a trusted battalion of District troops (if I remember rightly, it was the National Guard under Colonel Tait) formed in a semi-circle at the foot of the great stairway, and prevented all entrance from without. When the crowd began to assemble in front of the portico, a large number of policemen in plain clothes were scattered through the mass to observe closely, to place themselves near any person who might act suspiciously, and to strike down any hand which might raise a weapon.
Here’s an image from the inauguration, showing the platform mentioned by Stone and the masses that attended Lincoln’s swearing-in: