In his 1970 book, Historians’ Fallacies, David Hackett Fischer tried to classify the error represented by the old notion of a kingdom being lost for want of a nail in a horse's shoe.
He begins his essay explaining that all history involves the construction of causal strings and that falsity is more a matter of degree than anything else - a question of excess, really. He gives as an excessive example Churchill's remark (and it was only a comment) that "A quarter million persons died of that monkey's bite," referring to a king dying from a bite and thereby giving way to a successor who started a war.
Personally, that does not look like an extended causal string to me. Oddly enough, Fischer gives this expanded Civil War example as a more positive case:
[Lee's Lost Dispatch entered] the hands of the Union general George B. McClellan. A few Union special orders were then promptly issued, and there was a fight which the North called Antietam and the South called Sharpsburg ... It was the bloodiest day of the war, and a black one for Confederate arms. When it was over, General Lee was forced to retreat into Virginia. It is often said that Antietam was the decisive battle of the war. Many historians believe that it ended all chance of European intervention... Some are also of the opinion that this victory permitted Abraham Lincoln to gain a critical measure of control over his domestic opposition. Moreover, a few days after the engagement, Lincoln issued his preliminary Emancipation Proclamation. Are we to conclude from this story that the cause of Northern victory in the Civil war was the loss of Special Orders no. 191? The answer depends upon the causal model which is at hand. There is, I think, no prima facie case against the validity of such a causal interpretation, if it is understood that everything depends upon the acceptance of a contingent-series model of causality, and if the question at hand can be fairly and fully met with such an explanation.
These are interesting arguments, and the problem is interestingly named by Fischer "the reductive fallacy." Note that the type of contingency here depends on connections between activities or events.
There's a different kind of reductive fallacy much more prevalent in Civil War history that I want to talk about tomorrow.