AFTER the Sept. 11, 2001, terrorist attacks, it was widely asserted at the time, nothing would be the same. What Pearl Harbor had been for our parents and grandparents, 9/11 would be for us: a shattering national wake-up call revealing the gaping holes in America’s homeland security and the reality that we were at war with an implacable enemy whose defeat would require years of resolve.

But it became clear after a while that for many Americans, 9/11 had not marked a break with old ways of thinking. As the near-unanimity of 9/11 receded, Americans divided into what Fred Barnes dubbed September 12 people, for whom 9/11 had changed everything, and September 10 people, who believed the terrorist threat was being exaggerated by the Bush administration.

Would that divide have closed if Umar Farouk Abdulmutallab had succeeded in blowing up Northwest Airlines Flight 253 over Detroit on Christmas Day? If Al Qaeda, which reportedly trained Abdulmutallab in Yemen and is claiming responsibility for the thwarted attack, had succeeded in carrying out another 9/11, would the short-lived unity and moral clarity of that terrible day in 2001 have returned?

Had Flight 253 ended in mass-murder as the bomb plotters intended, Americans would today be filled with grief and fury. They would also be grappling with some hard lessons - lessons that in recent years too many had been inclined to dismiss. Among them:

1. Terrorism isn’t caused by poverty and ignorance. Abdulmutallab came from a wealthy and privileged family and had studied at one of Britain’s top universities. He wasn’t trying to kill hundreds of Americans out of socioeconomic despair. Like the 9/11 hijackers and countless other jihadists, Abdulmutallab was motivated by ideological and religious fanaticism. The teachings of militant Islam may seem monstrous to outsiders, but that is no reason to doubt that their adherents genuinely believe them or that by giving their lives for jihad they hope to change the world.

2. The global jihad is real. Homeland Security Secretary Janet Napolitano was widely derided for initially insisting that Flight 253 wasn’t blown up because “the system worked’’ and “the whole process went very smoothly.’’ But far more troubling was her effort to downplay any suggestion that Abdulmutallab’s attempted attack was “part of anything larger’’ - this even after he had acknowledged his ties to Al Qaeda. Of course Abdulmutallab is part of something larger: He is part of the global jihad - the relentless assault by Islamist radicals whose deadly serious goal is the submission of America and the West to Islamic law. If government officials like Napolitano cannot bring themselves to speak plainly about the jihadists’ ambitions, how will they ever succeed in crushing them?

3. Terrorists can always adapt to new restrictions. After 9/11, knives and sharp metal objects were banned from carry-on luggage, so Richard Reid attempted to detonate a shoe bomb. Thereafter everyone’s shoes were checked, so the Heathrow plotters planned to use liquid-based explosives. Now liquids are strictly limited, so Abdulmutallab smuggled PETN, an explosive powder, in his underwear. There is no physical constraint that determined jihadists cannot find a way to circumvent. Yet US airport security remains obstinately reactive - focused on intercepting dangerous things, instead of intercepting dangerous people. Unwilling to incorporate ethnic and religious profiling into our air-travel security procedures, we have saddled ourselves with a mediocre security system that inconveniences everyone while protecting no one.

4. The Patriot Act was not a reckless overreaction. Security in a post 9/11 world has not come from pressing a “reset button,’’ sending Guantanamo inmates to Yemen, or avoiding phrases like “war on terrorism.’’ It has come from stepped-up surveillance, stronger intelligence-gathering tools, and governmental authority to seize records, monitor communications, and conduct searches. Congress was not out of its mind when it enacted the Patriot Act in 2001, and the Bush administration was not trampling the Constitution when it deployed the expanded powers the law gave it: They were trying to prevent another 9/11, and by and large they succeeded. President Obama has repeatedly and ostentatiously criticized his predecessor’s approach. Perhaps it is not coincidental that Obama’s first year in office has also seen an unprecedented surge in terrorist threats on US soil.

We came fearfully close to having to re-learn those lessons the hard way last week. The world remains extremely dangerous, and the war against radical Islam is far from over. Flight 253 was another wake-up call. Did the September 10 people hear it?

Jeff Jacoby can be reached at jacoby@globe.com.

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