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During Abraham Lincoln's presidency, the Secret Service didn't exist. The President often took late night strolls through the streets of DC, or would give impromptu speeches to crowds without even a thought of having an armed guard. In fact, pretty much any human could walk on up to the White House and ask to have an audience with the President of the United States, and if the President's schedule wasn't too full that day, they were likely to have a 1-on-1 with the man himself. So, it was only a surprise to John Wilkes Booth that when he moved through Ford's theater there were no guards or soldiers there to impede him. The only man Booth encountered on his way to kill the POTUS was the White House Coach driver, who Booth briefly spoke to before going into the Presidential Box and putting a musket ball behind the left ear of The Great Emancipator.
Even though they hadn't thought to protect the leader of the country immediately after a Civil War, surely they realized their mistake following this attack, right? Nope. The Secret Service would not be created for another 36 years after President William McKinley was assassinated on September 6, 1901.
For more information:
Manhunt: The 12 day Chase to Catch Lincoln's Killer (Free with Amazon Prime or Scribd)