It so happens, I am reading a lot of what can only be called conspiracy theory just now and so am in pretty good shape to tackle the question.
First, let's take off the table the narrow Wikipedia-type definition, that conspiracy theory attempts to explain a chain of events originating in concealed causes. On its face, that would be speculative history, not conspiracy theory.
It seems to me that a conspiracy theory is an explanation relying on more than one hypothesis in a construct where the hypotheses inferentially support one another.
So, to use a familiar example, our examination of widespread reports that McClellan was on the field commanding at Gettysburg – that is not a conspiracy theory but a collection of reports tracking rumor dissemination. When we found one or more sources for these false reports, human nature invited us to ascribe motive to the officer spreading the story. Did he get it from somewhere? Misinterpret that someone wanted McClellan to command state militia? Put out a rumor to embarrass Meade and/or Hooker? It's a single choice and the reader should be able to spot it as iffy whether or not we label it "our hypothesis."
The conspiracy theory definition might come in if multiple disseminators were found to have some commonality: they were all enemies of Hooker or Meade … And yet, if we ascribe the McClellan rumors to "enemies of Hooker or Meade" we have still not crossed from speculative history into conspiracy theory until we strip our working hypothesis of uncertainty and begin to regard it, and present it, as definitive. We are making multiple inferences, making them co-dependent, and then proceeding with certainty. The conspiracy theory wishes to substitute for evidence in your evaluations.
So in addition to my definition, there is an operation that has to be performed to make it "conspiracy theory."
The libertarian economist Murray Rothbard used to distinguish between good and bad conspiracy theory. "Good" is a misnomer and of no interest here. He gave an economic example of the problem of "bad" conspiracy theory:
First, he [the theorist] stops with the cui bono; if measure A benefits X and Y, he simply concludes that therefore X and Y were responsible. He fails to realize that this is just a hypothesis, and must be verified by finding out whether or not X and Y really did so.Lenin, a conspirator himself, summed this up as the who/whom analysis. A variant of this is based on the "present at the same meeting" observation and that sometimes gets expanded to "present in the same city at the same time," and so forth.
Rothbard also noted "the bad conspiracy analyst seems to have a compulsion to wrap up all the conspiracies." Put more generally, one sees conspiracy theory, not recognizing itself as theory, trying to do way too much in the way explanation. Often, the authors, intrigued with their own insight, broaden the analysis and attach all sorts of things that don't belong even in a speculation.
Again, in addition to a working definition of conspiracy theory, we need a statement about conditions of use. I can present a multi-part speculation, all of it circumstantial and co-dependent, and still not have it be a conspiracy theory as long as I maintain the distinction in my mind and yours that we are speculating and I am offering my speculation for review and approval.
Often, instead, the speculator has become confused, considering a multipart, interdependent explanation as "decided" when it is not decided. We see this in politics, in life, and in Civil War history. At this point the real craziness begins as multiple speculative constructs are stacked one upon another. The fruit of this kind of pyramiding can be spotted by its outlandishness, say a La Rouche Democrat, for instance, asserting that the Queen of England runs drugs. There is no straight path to such conclusions.
What this has to do with Civil War history is (to me) interesting. You could generalize what I am about to say into pop history at large. In Civil War history, conspiracy theory can take serial form.
(1) I am currently reading an ACW battle book in which the writer attacks other writers for using force counts different from his own. He has no explanation for how he derived his comparative strengths. His strength numbers are a fixed idea.
(2) He places enormous pressure on his numbers. He uses them to derive opinions about the relative merits of contending commanders, about their efficiency, about their capabilities, and even about their moral character. He uses them to decide how quickly armies can be moved and he assumes all his numbers arrive on the battlefield and in good order. His unexamined number assumptions become hydra heads of forcefully stated misinterpretation.
(3) He takes the conclusions drawn from his number assumptions, about the character and capability of commanders, about opportunity won or lost, about skill, near misses, etc. and spins these up to a next level, into insupportable hard and fast conclusions about how the war should have been run, decisions the president should have made or enforced, generals who should have been hired or fired, etc.
The author stripped away the tentative character of his conclusions about certain numbers to build vertically (or serially) a construct that reaches conclusions as outlandish as "The Queen of England is a drug dealer."
There is another approach to achieving conspiracy theory status is Civil War history. By committing to a narrative and to firm explanations of ambiguous events, the writer digs himself ever deeper into a storytelling pit at the bottom of which lies absurdity. The narrative becomes a machine for generating confusion.
The demands of narrative history have generated surmises that are often not recognized as surmises downstream; when you get to meta analysis of the war, these surmises are then piled up to reach extremely speculative conclusions.
Single speculations serve as waypoints to advance a story line in a certain direction; after a few speculative choices, the narrator becomes committed to a set direction and that now affects his treatment of the next necessary speculation; he becomes ever more committed to a certain line or channel as the writing progresses. Chances are, he also wants to deliver confidence in his decisions, so choices are dressed up as unavoidable conclusions – the equivalent of facts. The reader may not even know he is reading speculation.
At the next level, the meta historian or compiler or aggregator takes these unstable narratives and begins spinning his metahistory. The result can be a highly respectable even prizewinning "conspiracy theory" that helps us misunderstand whatever factual data was offered.
I never tire of telling how Stephen Sears wrote confidently that R.E. Lee got his intelligence from newspapers. When Sears himself found timely newspaper accounts that McClellan found the Lost Order, Sears rationalized away Lee not reading those reports. When an amateur historian discovered additional timely newspaper accounts of McClellan's discovery of the Lost Order, he spent half an article presenting the new finds and half abjectly apologizing to Sears while warning readers not to discount Sears just because of an abundance of contrary evidence.
That would be one true fruit of conspiracy theory. There are many hereabouts.