For many people, the most revealing moment in last week’s debate among Republican presidential candidates came when Rick Perry was asked about his record on capital punishment in Texas. Before Perry even had a chance to speak, the audience burst into applause, simply at the recitation of the fact that Perry as governor has signed off on the execution of 234 people.

This isn’t about whether capital punishment is justified; that’s something that reasonable people can disagree on. There’s a big further jump to get to the point where executions are something to cheer about.

But that’s where today’s Republican Party finds itself.

As Steve Benen at the Washington Monthly put it: “This isn’t just wrong; it’s scary. The fact that Republicans in the audience found this worthy of hearty applause points to a party that’s bankrupt in more ways than one.”

On a day when everyone is talking about “the day that changed America”, it seems to me that this is the really significant change; that over a period of just a few years, starting roughly with the Clinton impeachment of 1998 and extending into the aftermath of 9/11, the Republican Party left the “reality-based community” behind and became the sort of organisation that cheers invasion, torture and executions.

That change is in turn the most important ingredient for explaining the oddly disproportionate reaction of the US to the events of 10 years ago.

The Western world had been subject to many terrorist attacks in the previous 40 years, from the Red Army Faction to the Munich Olympics, the IRA bombings in London, the Achille Lauro, Oklahoma City and many more. The 9/11 attacks were bigger and more spectacular, but they were not qualitatively different; only the response was.

What had changed was that those in charge of the US government had a predisposition to military solutions, to the language of terror and apocalypse. The idea of terrorism as an “existential threat”, so mysterious otherwise, came naturally to them. That’s why extreme measures — war, torture, detention without trial — were adopted not in the manner of regrettable necessities, but with enthusiasm, even with pride.

Hence the reluctance to even try to bring their actions within the ambit of international law; UN approval was not just unnecessary but positively undesirable. Hence the repeated disdain for the rule of law at home, and disregard for even basic standards of truth as well as justice. Hence the choice, still almost unbelievable, to attack Democrats for being “opponents of torture”.

The response to Perry shows that since losing office the Republicans have learnt nothing and forgotten nothing. The headlines may be about the deficit, health care and tax cuts, but the soul of the GOP is about lashing out violently at its real or imagined enemies.

Although I try to resist the temptations of hubris, I still think the best simple description of what happened is one I wrote nearly five years ago: “The US administration didn’t decide to throw away the rule book because it saw terrorism as an existential threat: it decided to portray terrorism as an existential threat because it wanted to throw away the rule book.”