There have been eleven assassination attempts on U.S. presidents. Four of those were successful. After William McKinley was killed in 1901, the secret service started being used for security and protection of the president, and less so for the task of checking for counterfeit currency, as they had been initially doing. Nowadays, you will be hard-pressed to see the president without a large amount of security surrounding him.
Learn more about the importance of protecting the POTUS and about the secret service in the infographic below, presented by Security Degree Hub.
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Security Degree Hub