About an hour after the north tower of the World Trade Center fell, on the way to stake out a friend’s apartment to learn whether he had escaped it (he had), I passed my neighborhood newsstand at East 77th Street and Third Avenue just as another man around my age was finishing a transaction.

“Hey, buddy,” the man said none too nicely to the normally affable but now visibly stricken Middle Easterner handing him change, “do yourself a favor; tell everyone you’re Puerto Rican from now on.”

That brief exchange surfaced to mind when news broke Monday of another terrorist attack in London — this time against Muslims. Apparently, a non-Muslim, almost certainly mimicking car attacks against Londoners for which ISIS has claimed responsibility, got into his van and careened purposely into a crowd of worshippers clustered outside a mosque. At least 10 people were taken to area hospitals; one man who was being treated at the scene for a medical emergency before the attack died.

What’s most surprising, in all candor, is that something like this hadn’t happened before. Revenge is a hard-wired human instinct and the United Kingdom has suffered a string of provocative terrorist attacks, including the appalling killing of children exiting an Ariana Grande concert last month. Males, especially, crave retribution when threatened; if anyone tells you otherwise he is lying.

But striking back against whom has been the emasculating question holding much of the western world together these past 16 years. In Syria and Afghanistan there’s an obvious answer. We are striking back at them — the guys holding Kalashnikovs across the field or caught hiding bomb-making matériel beneath their daughters’ cribs. But on the streets of London or on East 77th Street in Manhattan the who has been anything but clear.

The Islamic State and al-Qaida must have been over the moon upon learning of Monday’s terror event. It’s the violent reaction for which they’ve been agitating since before 9/11; the theoretical spark that will ignite all-out war between Muslims and non-Muslims on the streets of London, Paris, Amsterdam and New York. (ISIS targets Shia Muslims in this war as well.)

That fight is essential, in their doctrinaire minds, to prevent the assimilation of millions of Muslims into western society. Every burqa shed, every new Muslim Lions Club membership executed is a strike at the heart of fundamentalist Islam.

Excitement from Monday’s terror incident had to have been have been short-lived. Within hours of the mosque attack in London, came the democratic west’s headstrong counterattack: Brits of all backgrounds rallied around London’s Muslim community and decried the violence committed against it. (Take that!)

With some unfortunate exceptions, this antidotal dynamic has played out time and again in the face of modern terrorist agitation. A bomb goes off; a gunman opens fire, and for the most part peace between communities prevails. It’s a remarkable thing when one considers human nature.

Some invariably will see the London van driver as a hero in the war against terror. He is anything but; an unwitting tool of a cunning enemy. He mustn’t be emulated, but he very well could be.

Here now, especially, it would be stupid to fall for the terrorist bait. ISIS and al-Qaida have told us how to defeat them: integrate our Muslim-American neighbors as fully and expeditiously as possible through example and outreach.

There can’t be an “us and them” if there is only an “us.”

That genius of America will eventually prevail — if we stay smart.

William F. B. O’Reilly is a consultant to Republicans.