Today, the president of the United States and the Secret Service are inseparable—literally. Agents of the U.S. Secret Service accompany the president and the First Family everywhere and are particularly noticeable at public events. The agents also protect major presidential candidates. But it wasn’t always this way. It would take a third assassination of a U.S. president—William McKinley—to prompt Congress to assign full official protection of acting presidents.

The Secret Service was actually established in 1865 as a division of the United States Treasury that was primarily responsible for protecting the assets of the national treasury, safeguarding its currency production facilities and investigating counterfeiting. Beginning in 1894, Secret Service agents were protecting then-president Grover Cleveland, but only on a part-time basis.

Until then, and even in the years after, members of Congress were loath to formally establish a national law enforcement agency, preferring to leave functions related to law and order to individual states. However, it was when Cleveland’s successor, William McKinley, was assassinated in 1901 that momentum for such a federally run agency began to build.

“McKinley’s assassination came at a critical time in U.S. history,” notes Cary Federman, associate professor in the Department of Justice Studies, Montclair State University in New Jersey and author of The Assassination of William McKinley: Anarchism, Insanity, and the Birth of the Social Sciences.

“Up until then, there was a general hostility toward centralized power, and remaking the Secret Service as a national police force would, in effect, be taking some law enforcement power away from the states. However, after McKinley was killed, the incident became a factor in establishing the role of the Secret Service in protecting the president. There was a recognition of the president as a target.”

Still, Federman emphasizes that there were other factors as well. Indeed, in the aftermath of the assassination of Abraham Lincoln in 1865 and James A. Garfield in 1881, bills designed to formalize the role of the Secret Service in the protection of the president failed to gain approval in Congress. So what was different about McKinley’s death?

The fact that McKinley was the third president assassinated while in office over a 36-year period was certainly significant. It was also at this time that the United States began to emerge as an imperial power, and leaders at the national level started to become more concerned about political threats both inside and outside the nation’s borders.

Since the 1870s, anarchists had been staging attacks against governments and law enforcement personnel all over the world, including during the Haymarket Riot in Chicago in 1886. Although there is evidence to suggest that McKinley’s killer, Leon Czolgosz, was more mentally ill than politically motivated, he was labelled an anarchist in the immediate aftermath of his actions.

Theodore Roosevelt, who rose to the nation’s highest office after McKinley’s death, even said at the time, “When compared with the suppression of anarchy, every other question sinks into insignificance.”

According to Federman, Roosevelt was also a “strong nationalist,” meaning he believed in the centralization of powers. That, coupled with the perceived political and social threats facing the country at the time, motivated Congress to act and formalize the Secret Service’s role in protecting the head of state.

In 1902, within months of McKinley’s death, a Secret Service detail was assigned to guard the president of the United States 24-7-365. And that remains the norm to this day.

“In the popular view of the time, Lincoln’s assassin, John Wilkes Booth, was ‘one of us,’ a white American,” Federman explains.

“But Czolgosz? He had this ‘mysterious-sounding’ name. Even though he too was American. So yes, while McKinley’s death highlighted this idea of the president as a target, it was really the beginning of the United States being concerned about possible threats against it, both inside and outside the country, from ‘dangerous foreigners.’ That as much as anything else really became the justification for the creation of the Secret Service.”