Yet at that very moment in the American South, Butler noted, a domestic organization, the Ku Klux Klan, was murdering blacks and whites who supported the post-Civil War program of racial equality known as Reconstruction, and the Klan was operating with virtual impunity.
"Can it be," Butler demanded of those who insisted crime-fighting should be left up to the states, "that an American citizen is protected with the whole power of the government, everywhere, except on our own soil, under his own roof-tree, and covered by our own flag?"
Congress duly passed the Ku Klux Klan Act, and President Ulysses Grant signed it into law on April 20, 1871. The federal government could suspend the writ of habeas corpus in Klan-dominated areas and use federal marshals and troops to arrest Klan terrorists and bring them to trial in federal courts.
To this day, it remains arguably the toughest federal law ever aimed specifically at a white supremacist terrorist threat. It launched what might be called the first U.S. war on terror, including the use of covert operatives - detectives from the Secret Service - to infiltrate the Klan.
As if to mark the occasion, congressional Democrats, led by Sen. Richard Durbin, D-Ill., have just introduced the Domestic Terrorism Prevention Act, whose premise, like Butler's, is that terrorism is terrorism - whether it originates at home or abroad, on the racist right or the Islamist fringe.
Recent events, from the 2015 murder of nine African-Americans by a white supremacist in Charleston, South Carolina, to the massacre of Jews at a Pittsburgh synagogue last October, lend credence to that notion.
Though the body count is alarming, we are not witnessing violence on the scale of Reconstruction; Durbin's proposed measures are accordingly modest. Basically, his bill boosts training efforts, while requiring the federal government to gather intelligence on white supremacists and neo-Nazis, including any infiltration of law enforcement and the military. There would be annual reports assessing the threat.
The provision of the 1871 Klan Act it most resembles is one calling for Congress to impanel a special committee of investigation. Still, information is power; rebutting pro-Klan propaganda with the truth, gathered by the committee, was crucial to the Grant administration's success against the organization.
Grant's determination to defeat the Klan, based both on his morality and on his political interest in protecting Southerners, black and white, who were likely to vote for him, proved decisive in that long-ago struggle.
Conversely, there is a lack of presidential leadership regarding today's threat from the right - which President Donald Trump described, in the wake of the New Zealand mosques massacre, as "a small group of people that have very, very serious problems."
Trump calls to mind not Grant but his predecessor, Andrew Johnson, who used his bully pulpit to issue apologia for ex-Confederates and to play down outbreaks of violence against African Americans.
Another lesson of history is that an effective crackdown on white supremacist violence today, if and when it comes, may require tactics similar to those used in the U.S. government's recent "war" on other forms of terrorism.
The Reconstruction-era Klan used elaborate oaths, secret passwords and hand signals to keep law enforcement at bay, just as today's ultra-rightists communicate via encrypted electronic messages and chat rooms on the "dark Web."
U.S. government agencies will have to use surveillance and infiltration to penetrate these groups, just as Secret Service men, using skills previously honed fighting counterfeiters, donned disguises and spied on Klan units.
The Secret Service's chief, Hiram Whitley, had no qualms about any tactics to defeat lawbreakers, believing, as he put it, that "any strategy resorted to by the officers to bring them to justice is in my judgment perfectly justifiable."
This was Whitley's answer to those who questioned the constitutionality of federal covert operations against American citizens; a deep historical irony is that Southern propaganda against the Grant administration crackdown on the Klan drew heavily on the American civil-libertarian tradition.
By and large, the federal government today does possess the necessary means to address white supremacist and neo-Nazi terrorism within the law, and without violating constitutional boundaries. The missing ingredient is not the ruthlessness of a detective Whitley, but the commitment.
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