MacCarthyism in the US was a witch hunt, trying to find evidence of Communism in celebrities. Charlie Chaplin was under suspicion and left the country in the end, so did Sam Wanamaker and several other high profile people in the entertainment industry.
If you look at the public interrogations, you can see how awful it was. They believed Chaplin could have been a member of the Comunist party due to his poverty stricken background - his father was an alcoholic and walked out leaving his mother with mental health issues and they all ended up in the workhouse. It was his brother Sidney who saved them and Charlie never forgot either him or his mother and looked after them until they died.
MI5 found nothing untoward, apart from the fact that his birth wasn't registered. but lots of births weren't registered at the time. as several of my ancestors would have attested to!
Paranoid Americans again!!