It’s not really a conspiracy theory because it is codified in the 13th Amendment which reads:
“Section 1. Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.
Section 2. Congress shall have power to enforce this article by appropriate legislation.”
Because you refer to a “continuation of slavery,” I’m going to assume that you are focused on prisons as a continuation of the enslavement of African Americans, and focus the rest my response there.
That clause “except as a punishment for a crime...” is the subject of many works by contemporary legal and Africana scholars who assert that this caveat has been used since the earliest days of the 13th Amendment to allow a form of legalized slavery which encouraged the criminalization of blacks in the South who were not allowed to sit on juries and thus almost always convicted of any charges brought against them.
One of the early programs that most closely resembled slavery was convict leasing. This is a program that allowed the state to lease its convicts to private companies. It’s pretty damning as a “conspiracy” by the Southern states to continue having access to a virtually free labor force that was primarily African American (ie practically slavery) for a few reasons:
For more on this, you should read Douglas A. Blackmon’s book “Slavery by Another Name: The Re-Enslavement of Black Americans from the Civil War to World War II.”
The other post’s author gets in to the affects of the drug war on modern prison populations but i would like to expand on it a little bit.
There is some evidence (Edit: though it’s veracity is debated) from an interview with a former Nixon top adviser that the War on Drugs was essentially perpetrated as a means to harass “Blacks and hippies.” And whether or not this is true, it has definitely turned out that way for the former. Though white and black people use marijuana at the same rate, a black person is 3.73 times more likely to be arrested.
https://www.aclu.org/issues/smart-justice/war-marijuana-black-and-white
And overall, African Americans are disproportionately likely to be subject to stops and searches, arrests, prosecutions and plea negotiations, trials, and sentencing (20% longer sentences on average than comparable crimes committed by white offenders). It also affects the availability of plea agreements, likeliness of a life without parole sentence, likeliness of a death sentence, enforcement of school zone sentencing laws (which can drastically increase sentences), as well as the famous 100 to 1 disparity in mandatory minimum sentencing for crack vs cocaine.
The intentionality of this system (the “conspiracy” element of your question) is debatable. Outside of the contested quote by the Nixon advisor, the evidence of racist intent in the formation of these racially disparate systems is sparse even if their impact is clear.
I suggest checking out “Thirteenth” on Netflix. It does a great job of laying out this progression from slavery to the modern “school-to-prison” pipeline. (Edit*: For the modern prison system, it can present statistics in a way that is misleading or sometimes inaccurate but most of these issues are limited to the final section on crime policy since Clinton not the historical sections, and not the progression of slavery to the modern school-to-prison pipeline as suggested. Yes, it is a documentary with an agenda but it’s history for the era between the Civil War and the mid 90s is useful for a quick overview even if it is a little light on explaining the use of prisons during the Civil Rights movement.)
It absolutely has "credible historical merit," as you put it.
One could certainly read the Tennessee State Constitution as directly permitting slavery as punishment for a crime. See Constitution of Tennessee, Art. 1, s 33 (1870).
That slavery and involuntary servitude, except as a punishment for crime, whereof the party shall have been duly convicted, are forever prohibited in this state.
This provision was used as the basis of the state's "Convict Labor" system, largely the work of Nashville publisher and political boss Arthur St. Clair Colyar and his cronies in 1871. They obtained a lease on convicts so that companies run by these men could benefit from the free labor (or un-free labor, depending on your point of view). Notably, these convicts worked for the Tennessee Coal, Iron and Railroad Company (aka TCI, later part of US Steel). TCI's founder and principal investor was none other than Arthur St. Clair Colyar. However, the lion's share of TCI's convict leasing occurred in Alabama, and the practice continued there until 1928.
Colyar was nakedly unapologetic about his motivations for engaging in the convict lease system: it was cheap, strike-proof labor. Immediately after the American Civil War, the decimated southern states began rebuilding. Much of the economic activity has been described as "exploitative and colonial." The southerners who found jobs worked for out of town management, and paid rent to absentee landlords. A northern business magnate could make a tidy sum selling materials with which to rebuild a typical town square, and might well end up owning the buildings outright. The profits, however, were invested on Wall Street, not reinvested in Main Street. Local raw materials were not processed locally, but taken by the carload to distant (i.e. northern) points for preparation and sale. The nature of this activity resulted in depressed wages, and workers often went on strike not to gain higher wages, but to retaliate against threatened wage cuts. These strikes were occasionally successful, but frequently violent, even deadly affairs.
Rich deposits of coal and iron were being mined near Gadsden, Alabama, all the way up to Johnson City, Tennessee and beyond. Rented convicts, the majority of whom were black, displaced the mostly white miners. Many of the black convicts were born after the Civil War and thus, were never slaves. They absolutely resented being treated as second-class citizens when they rightly knew that they had been born free. Black men and women bristled under these so-called "black codes," violations of which could result in a prison sentence and "hard labor." In Nashville, able-bodied black men were shipped off to rail construction camps or coal mines in East Tennessee. By the late 1880s and early 1890s, reform-minded state legislators inspected these camps, where the filthy conditions were shockingly deplorable. One report described them as "hell-holes," and another declared them to be "a shame upon the state."
In July of 1891, the recently unemployed miners of Briceville, Tennessee were fed up. They surrounded the convict stockade and demanded that the guards surrender at once. The guards and their prisoners retreated to Knoxville by train, 30 miles southeast. Several days later, Governor J.P. Buchanan, personally commanding the state militia, marched the convicts back to Briceville. The miners stood their ground and on July 20, the convicts and militia retreated back to Knoxville. The miners then marched to Coal Creek (now Lake City Rocky Top) and harassed the prison guards and convict laborers there. Facing the threat of insurrection, Governor Buchanan called up 600 state militia, but the miners wouldn't budge. That is, until a representative of the Governor promised that a special session of the State Legislature would be called to reconsider the state's convict lease program. They reconsidered it alright, with so many members in the pockets of wealthy interests, the laws regarding the convict lease system were strengthened, not repealed.
The miners resorted to guerilla tactics: burning camps, freeing convicts, and even shootouts with the state militia. The state eventually conceded defeat when they realized that they had spent $200,000 ($5.4 million today) in state revenue on pay for the militia, but earned only $75,000 ($2.1 million today) from the convict leasing fees. With continued unrest and increasing unpopularity, the Tennessee General Assembly repealed the convict lease program in 1893, and built a new prison in 1896 (Brushy Mountain, closed in 2009) where convicts would mine coal for direct sale by the state. What is now called the "Coal Miner's War" was now over. That same year the last of the TCIs convict leases expired.
Source: Bergeron, et al., Tennesseans and their History University of Tennessee Press, Knoxville (1999), pp 199-201.
As we can see, the US prison system is, when viewed through this prism, a continuation of slavery.
edit: fixed typos.
The easiest way to trace slavery's legacy in the modern American penal system is through convict leasing. Others have talked about it briefly, so I'll add only a little information.
"Convict leasing," a system whereby the state "leased" incarcerated people to private enterprise to be used for labor, had many of the trappings of slavery, with people of color placed in bondage and used for the benefit of (white) landowners.
A brief summary of the practice, what it looked like, and why it happened:
With certain political and economic advantages to recommend it, convict leasing served as "the mainstay of the penal most southern states" in the post-Civil War era. Those who the prisoners kept the convicts hidden from public view lease camps and so removed them from the public consciousness. The convicts provided a good source of labor to rebuild the structure of the South that the Civil War had demolished. And while the system spared the state the expense of convict care, in some cases convict leasing actually made a profit. Because many southern states required their prisons to be financially self-sufficient, the profit-making convict leasing system survived despite certain serious problems which emerged shortly after the system was put into place.
From Arkansas Historical Quarterly, "The Long Struggle to End Convict Leasing in Arkansas."
It might be tempting to brush this off, assuming, well, these people were convicted of crimes so who cares what the state did with them? But the criminal codes of the post-war South were devised with the deliberate goal of increasing black incarceration, and that's exactly what they achieved. Michelle Alexander covers this topic in her book, The New Jim Crow -- pages 28-29. (If you come across someone inclined to discount her as an activist rather than a historian, she footnoted her book very thoroughly and you don't have to take her word for it.) These new "black codes" were established for the secondary, if not primary purpose of "control[ing] the freedmen" -- a quote from one of Alexander's sources, not her writing -- and achieved that goal by criminalizing homelessness and unemployment, two things that ran rampant among the just-freed black population of the South.
Fast forward to the 1950s or so and much of that infrastructure remained in effect. Even after the abolition of convict leasing, and the dawn of the civil rights era, Southern states often balked at investing in humane prison systems and forced their (still predominantly black) incarcerated population to farm for their own subsistence and even police themselves, making the early modern southern prison look a lot like a slave plantation. This was especially true in Arkansas, where the two main state prisons, Cummins Farm and Tucker Farm, implied by their very names that incarcerated people were treated little better than slaves. You'll find a thorough description of those conditions in Malcolm Feeley & Edward Rubin, Judicial Policy Making and the Modern State: How the Courts Reformed America's Prisons, pages 45-55.
Those systems were broken up by a spate of 1950s and 1960s-era civil rights litigation, so the legacy of slavery remained a very real force in American corrections into the recent past. (You can read more about that in a forthcoming book chapter I wrote with the American Enterprise Institute!) And arguably the legacy still lives on today, where people of color are disproportionately policed and incarcerated, but that might be outside the scope of the question!