The canonical example is WWii Germany. Denazification actually sort of worked. But it required a lot of effort, resources and special circumstances.
Yes a bunch of previous nazis made it back into power and politics, but they didn't call themselves nazis or acted like nazis. But also, the country as a whole took a very different path after wwii.
Justice was never served for what the Nazis did. Both the US and the USSR scooped up Nazi scientists (Operation Paperclip), and with the advent of the Cold War, the West quickly decided that it cared more about contesting Europe with the Soviets than seeking justice.
(1945 - 1949 it was split in 4 occupation zones)