The fallacy of reductio ad Hitlerum is a specific form of the guilt by association fallacy, wherein the specific entity associated with the target is Adolf Hitler, or the Nazis in general.

Here is an example:

1. Hitler favored putting prisoners to work in fire brigades.

2. Therefore, putting prisoners to work in fire brigades is wrong.

The problem here is that Hitler's support of something does not guarantee it is wrong. If Hitler asserted "2 +2 = 4", it would not change the correctness of that statement. So, a statement must be tested for truth independently on whether Hitler supported it or not.

​The fallacy is the same if "Hitler" is replaced with any other infamous historical person or group, e.g. Mussolini, slave-owners, etc.