Mills didn't live through fucking First and Second World War.
Less well known is the paradox of tolerance : Unlimited tolerance must lead
to the disappearance of tolerance. If we extend unlimited tolerance even to
those who are intolerant, if we are not prepared to defend a tolerant society
against the onslaught of the intolerant, then the tolerant will be destroyed,
and tolerance with them. — In this formulation, I do not imply, for instance,
that we should always suppress the utterance of intolerant philosophies ; as
long as we can counter them by rational argument and keep them in check
by public opinion, suppression would certainly be most imwise. But we
should claim the right to suppress them if necessary even by force ; for it may
easily turn out that they are not prepared to meet us on the level of rational
argument, but begin by denouncing all argument ; they may forbid their
followers to listen to rational argument, because it is deceptive, and teach
them to answer arguments by the use of their fists or pistols. We should
therefore claim, in the name of tolerance, the right not to tolerate the intolerant.
We should claim that any movement preaching intolerance places itself out-
side the law, and we should consider incitement to intolerance and persecution
as criminal, in the same way as we should consider incitement to murder, or
to kidnapping, or to the revival of the slave trade, as criminal.
Popper, Karl R. The Open Society and Its Enemies: Volume 1, The Spell of Plato. Routledge, 1945