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The historian's fallacy is a logical fallacy that occurs when one assumes that decision makers of the past viewed events from the same perspective and having the same information as those subsequently analyzing the decision. It is not to be confused with presentism, a mode of historical analysis in which present-day ideas (such as moral standards) are projected into the past.

The fallacy was named in 1970 by David Hackett Fischer, who suggested it was analogous to William James's psychologist's fallacy. Fischer did not suggest that historians should refrain from retrospective analysis in their work, but he reminded historians that their subjects were not able to see into the future. As an example, he cited the well-known argument that Japan's surprise attack on Pearl Harbor should have been predictable in the United States because of the many indications that an attack was imminent. What this argument overlooks, says Fischer, is that there were innumerable conflicting signs which suggested possibilities other than an attack on Pearl Harbor. Only in retrospect do the warning signs seem obvious; signs which pointed in other directions tend to be forgotten. (See also: hindsight bias).

In the field of military history, historians sometimes use what is known as the "fog of war technique" in hopes of avoiding the historian's fallacy. In this approach, the actions and decisions of the historical subject (such as a military commander) are evaluated primarily on the basis of what that person knew at the time, and not on future developments that the person could not have known. This technique was pioneered by the American historian Douglas Southall Freeman in his influential biographies of Robert E. Lee and George Washington.

Other examples of the historian's fallacy:

• Julius Caesar should not have seized power because it would lead to his assassination.
• World War I should not have been fought because it accomplished nothing and led only to World War II.

• Fischer, David Hackett. Historians' Fallacies: Toward a Logic of Historical Thought. New York: Harper Torchbooks, 1970.