Brainwashing actually has been quite successful over the years. Thus far, we only know of a few instances of confirmed brainwashing that caused some big changes. The reason this can be hard to do is that it has to be a systematic overhaul of a person’s beliefs and what they know to be true. It’s like if someone told you that your name is not what it is. You know your name is what it has been and believing anything other than that was not possible.
However, it has happened. Memorably, this occurred during the height of the Korean War. Colonel Frank Schwable was shot down and captured. He along with several other prisoners of war made false claims that the U.S. used germ warfare against Korean people. They claimed America dropped anthrax on them and much more. 21 American soldiers refused to return to the nation as well. This proved brainwashing had to have occurred. Zombie brainwashing anyone?