The historian's fallacy occurs when arguer evaluates a historical event as if its participants had knowledge that was available only after the event had transpired.

The classic example is interpreting the Japanese attack on Pearl Harbor in WWII as "predictable" in view of "warning signs" that US military officials allegedly "should have seen."

In actuality, there were many conflicting data points suggesting a variety of possibilities of what Japanese forces were about to do, other than attacking Pearl Harbor. Some of the data was deemed "obvious" only after the attack had occurred, when all the other conflicting information was promptly ignored or forgotten -- creating the false impression that it "should have been predicted."