Answer: They all emerged from the murky depths of leftist politics. And all have a history of violence associated with their politics.
The Klan, which came to life after the Civil War, has been accurately described by Columbia University historian Eric Foner as “a military force serving the interests of the Democratic Party.” University of North Carolina historian Allen Trelease has similarly described the Klan as the “terrorist arm of the Democratic Party.” PBS.org has this recounting of the Klan’s violent history:
Originally founded as a social club for former Confederate soldiers, the Klan evolved into a terrorist organization. It would be responsible for thousands of deaths, and would help to weaken the political power of Southern blacks and Republicans.
Racist activity in the South often took the form of riots that targeted blacks and Republicans. In 1866, a quarrel between whites and black ex-soldiers erupted into a full-fledged riot in Memphis, Tennessee. White policemen assisted the mobs in their violent rampage through the black sections of town. By the time the violence ended, 46 people were dead, 70 more were wounded, and numerous churches and schools had been burned. […]
In this violent atmosphere, the Ku Klux Klan grew in size and strength.
Move on to the left-leaning labor movement and here is Morgan O. Reynolds, an associate professor of economics at Texas A&M University, who explains the gross justification of union violence often seen from the Left: “Union violence is exciting in and of itself for many intellectuals, who generally are bored by stability and gradual material progress. Labor violence also provides intellectuals with support for their view that workers are alienated from the economic system.”
Move from the labor movement to 1960s and ‘70s America and there appears the Left’s Students for a Democratic Society and the Weather Underground. In a paper at the American Public University, Scott Midgley writes:
The 1960’s in America were a turbulent and chaotic time. Perhaps no group epitomized the cultural, social, and political clashes that were occurring more than the Weathermen, also known as the Weather Underground. Almost instantly, they became one of the most notorious domestic terror groups in America, responsible for dozens of bombings in the name of revolution. Splintering from the Students for a Democratic Society in 1969, the Weathermen espoused violent rhetoric; their goal was to create a clandestine revolutionary party for the violent overthrow of the United States government ("State Department bombing,"1975). They sought to create a "white fighting force" to be allied with the Black Liberation Movement, as well as other radicals, to achieve "the destruction of U.S. imperialism and achieve a classless world: world communism." (Asbley et al., 1969).
Move ahead from the 1960s to the early 21st century and the appearance of the Left’s Occupy Wall Street — a movement endorsed by President Obama, then-Speaker Nancy Pelosi and, yes indeed, ex-Klan leader David Duke. Over at the Powerline blog, John Hinderaker wrote in 2012:
The Occupy movement has a horrific record of violence. Occupy Wall Street Exposed has counted a dozen deaths, including three murders; more than a dozen rapes; more than 25 disgusting cases of indecent exposure, public defecation, etc; more than 500 thefts; more than 6,800 arrests; and in excess of $12 million in property damage. A sorry record by any standard.
But the would-be terrorist attack that was foiled on May Day, in which five Cleveland Occupiers plotted to blow up a bridge, was something different–an act of domestic terrorism.”
Back one year earlier, in 2011, a Seattle Times story began:
Seattle police say the level of open violence toward officers at Monday’s Occupy the Port protest hasn’t been seen since the World Trade Organization was here in 1999.
"It’s been a long time since we’ve seen people throwing things that could kill people, not since WTO," said Asst. Chief Mike Sanford at a news conference Thursday. "We’ve seen debris thrown, but … not bricks, not sharpened rebar, not things that would kill somebody.”
Now comes James Hodgkinson and his shooting spree that seriously wounded Republican Congressman Steve Scalise and injured others at a practice session for the annual Congressional Baseball Game. Reports the New York Daily News:
While cops declined to provide a motive in the shooting of Rep. Steve Scalise (R-La.) and three others, the 66-year-old brought along a resume of liberal activism and anti-Trump rhetoric on his journey.
Photos show he kept a white poster inside the van with a cartoon image of Trump. It read: “Chicken in Chief.”
…He belonged to a Facebook group with the foreboding name "Terminate the Republican Party." And one of his Facebook postings three months ago took direct aim at President Trump: "Trump is a Traitor. Trump Has Destroyed Our Democracy. It’s Time to Destroy Trump and Co."
A 2015 post actually mentioned Scalise, the lone congressman wounded in the shooting spree.
So, what do we have here?
What we have is a very long and very consistent pattern of violence from leftist groups or individuals. The subjects and target can change — from murdering blacks, to labor violence, to protesting the Vietnam War, to rampaging through the streets of Seattle for Occupy Wall Street, to shooting Republican members of Congress. Time comes and goes, from the mid- and late-1800s all the way through the 20th century to early 21st century.
If, in fact, there is to be a serious conversation in America about political violence after the Scalise shooting, there needs to be a candid and crystal-clear recognition that the American Left (and, for that matter, the Left around the world) does in fact have a proclivity for violence. That violence is in the Left’s DNA and has manifested itself repeatedly across (or parts of) four — say again, four — centuries.
The question now? What to do about it.
And the very first step in dealing with it is to recognize the pattern.