What did the FBI have against John Lennon?

I'm watching something on TV about the declified FBI files on John Lennon, and I don't understand what the big deal was. They keep saying he was "dangerous" because he was a war protestor. So what, he protested the war. There's a lot of people today that protest the war in Iraq, but that doesn't mean they want to overthrow the government. John Lennon was a musician, not a politician. He may have been against the war in Vietnam, but so were many people in that time. So what's wrong with protesting a war? If you protest the war, the FBI clifies you as dangerous??