WASHINGTON – In 1977, when a bunch of neo-Nazis decided to march through Skokie, a suburb of Chicago heavily populated with Holocaust survivors, there was controversy as to whether they should be allowed. I thought they should. Why? Because neo-Nazis are utterly powerless.

Had they not been – had they been a party on the rise, as in late-1920s Germany – I would have been for not only banning the march, but for practically every measure of harassment and persecution from deportation to imprisonment. A tolerant society has an obligation to be tolerant. Except to those so intolerant that they themselves would abolish tolerance.

Call it situational libertarianism: Liberties should be as unlimited as possible – unless and until there arises a real threat to the open society. Neo-Nazis are pathetic losers. Why curtail civil liberties to stop them? But when a real threat – such as jihadism – arises, a liberal democratic society must deploy every resource, including the repressive powers of the state, to deter and defeat those who would abolish liberal democracy.

Civil libertarians go crazy when you make this argument. Beware the slippery slope, they warn. You start with a snoop in a library, and you end up with Big Brother in your living room.

The problem with this argument is that it is refuted by American history. There is no slippery slope, only a shifting line between liberty and security that responds to existential threats.

During the Civil War, Lincoln went so far as to suspend habeas corpus. When the war ended, America returned to its previous openness. During World War II, Roosevelt interned an entire ethnic group. His policies were soon rescinded (later apologized for) and shortly afterward America embarked on a period of unprecedented expansion of civil rights. Similarly, the Vietnam-era abuses of presidential power were later exposed and undone by Congress.

Our history is clear. We have not slid inexorably toward police power. We have fluctuated between more and less openness depending on need and threat. And after the 9/11 mass murders, America awoke to the need for a limited and temporary shrinkage of civil liberties to prevent more such atrocities.

Britain is just now waking up, post-7/7. Well, at least its prime minister is. His dramatic announcement that Britain will curtail its pathological openness to those who would destroy it – by outlawing the fostering of hatred and incitement of violence and expelling those engaged in such offenses – was not universally welcomed.

His own wife had made a speech a week after the second London bombings, loftily warning against restricting civil liberties. “It is all too easy to respond in a way that undermines commitment to our most deeply held values and convictions and cheapens our right to call ourselves a civilized nation,” declared Cherie Blair. You need only read Tony Blair’s 12-point program to appreciate how absurd was his wife’s defense of Britain’s pre-7/7 civil liberties status quo.

For example, point 3: “Anyone who has participated in terrorism, or has anything to do with it anywhere will be automatically refused asylum in our country.” What sane country grants asylum to terrorists in the first place?

Point 5, my favorite, declared “unacceptable” the remarkable fact that a man accused of the 1995 Paris metro bombing has successfully resisted extradition across the Channel for 10 years.

Blair’s proposals are progress, albeit from a very low baseline – so low a baseline that the mere announcement of his intent to crack down had immediate effect. Within three days, the notorious Sheikh Omar Bakri, a Syrian-born cleric who has been openly preaching jihad for 19 years, skipped the country and absconded to Beirut.

Not only had Bakri been allowed to run free the whole time, but he had collected more than 300,000 pounds in welfare, plus a 31,000-pound gift from the infidel taxpayers: a Ford Galaxy (because of a childhood leg injury).

It took 52 dead for at least the prime minister to adopt situational libertarianism. Or as Blair put it, “The rules of the game are changing,” declaring his readiness, finally, to alter the status quo in the name of elementary self-defense.

Before departing Britain, Bakri complained that it would be unfair to have him deported from the country he reviled: “I have wives, children, sons-in-law, daughters-in-law. It would be hard on my family if I was deported.”

Wives, no less. Point 10 of Blair’s plan would establish a commission to try to get immigrants to adopt more of the local mores.

Charles Krauthammer is a Washington Post columnist. Contact him by writing to letters@charleskrauthammer.com.