What is the lesson of Free Speech that was understood by everyone who went through World War II but not now?

The problem is "Words can Kill!"

So, what people learned in World War II is that if you let people speak however they want about any issue it was less likely they were going to get pissed off so much they started killing people.

Repressing Free Speech causes Wars and deaths. So, whenever you repress the free Speech of Liberals, Moderates or Conservatives around the world you are creating a situation often where people are going to die because of this repression.

There is a saying of "The Writing is on the Wall!"

which means that people if repressed tend to scrawl what they really think upon the walls of buildings and elsewhere. You really cannot repress the human spirit no matter what unless you killed Everyone on earth of all political points of View.

We live in dangerous times and it was just as dangerous for Democrats to repress Free Speech in regard to many things as for Republicans to repress now the free Speech of Liberal Democrats.

Both lead to a civil War which we see playing out now in various ways across the country.