The reductio ad Hitlerum is a popular rhetorical technique for dismissing others’ arguments without much reason. In City Journal Dalrymple specifies some of the many problems stemming from its use:
The reductio ad Hitlerum is an argument from historical analogy, and analogy is, by definition, always inexact; otherwise, it would be repetition. As no less a person than Karl Marx put it, “history repeats itself, first as tragedy, second as farce”—that is, it does not, and cannot, repeat itself exactly. While analogical historical reasoning cannot be altogether eliminated, therefore, it must be used with judgment, discretion, discrimination, and care. History teaches neither nothing nor everything; and it is as dangerous to use it wrongly as to disregard it altogether.