Sixteen years ago today a war was brought to our city streets by an enemy we already knew but misunderstood. The 9/11 attack may have been the ultimate “black swan” event: something that we should have known was coming and should have prepared for but didn’t.

There are many reasons for the failures to foresee and to prepare, some of which still apply to us today. They are reflected in our cultural attitudes, the way we have fought the war, and in the people we have chosen to be president since September 11. 2001.

They form the lessons we should have learned from this war and still haven’t.

Osama bin Laden declared war on America in his religious “fatwa” published by a London newspaper in 1996. It called upon every Muslim to kill Americans anywhere they are found and said that his followers had no intention except to enter paradise by killing us.

Our intelligence agencies knew not only of his intent but of his growing ability to carry out his goals. They knew of the training camps that the Taliban used to help train bin Laden’s men. So much was known and ignored. As the Twin Towers burned on 9/11, sources were pouring out information that bin Laden was behind the attacks. I wrote as much in a column that appeared in the Washington Times on September 12, 2001.

Our war against the Taliban began on October 7, 2001 and it continues to this day. Why?

Two presidents — Messrs. Bush and Trump — adopted strategies to try to defeat them and to support the Afghanistan government while our troops fight the Taliban. President Obama only compounded Mr. Bush’s mistake in nation-building, kicking the can down the road so that his successor would have to deal with Afghanistan and Obama wouldn’t be blamed for “losing

President George W. Bush had said he was opposed to nation-building during his 2000 campaign, but he immediately reverted to it in Afghanistan and continued it in Iraq. Why?

Mr. Bush, because he had become a neoconservative, believed that everyone, everywhere, had the most fundamental desire to be free in the same way Americans are. He was fundamentally wrong. The Muslim culture, ingrained in believers for about fourteen hundred years, doesn’t allow the freedoms we enjoy in any way, far less as we enjoy them.

That is the most basic of the Unlearned Lessons of this war. Muslims, ideologically and culturally, are different from Christians and Jews in this most fundamental way. It’s why we weren’t — as Mr. Bush expected — welcomed as liberators in Afghanistan and Iraq.

The Taliban have never been defeated and the Iraqi terrorist networks, both Sunni and Shiite, have not been either.

Mr. Bush was convinced that Afghanis and Iraqis would welcome the arrival of the new freedoms we sought to give them, but they didn’t. Their religion, and the culture derived from it, made us the enemy regardless of what we did.

The second unlearned lesson is another of Mr. Bush’s mistaken beliefs. He believed that nation-building succeeded in Japan and Germany after World War II — and it did — but only because the enemies there had been utterly defeated. The ideology of Nazism died with Hitler and his cronies. The Shinto religion wasn’t destroyed: it thrives, alongside Buddhism, in today’s Japan. But Japan’s utter defeat defeated the ideology of Shintoism that drove Imperial Japan to its conquests. Mr. Bush rejected the idea of fighting the ideological war that the Islamist wage against us.

The lesson of World War II — and the Korean War, and Vietnam — is that if you don’t fight a war in a manner calculated to win it decisively, you will lose it inevitably. Unless an enemy is as defeated as the Japanese and Germans were in that war, the war can go on indefinitely. And it has.

Nevertheless, President Trump’s “new” Afghanistan strategy will continue nation-building despite the fact that the Taliban, though beaten many times, are undefeated. Their ideology, Islamism, remains as strong in Afghanistan as it was before 9/11. Mr. Trump leaves U.S. forces to fight the terrorists, but still devotes lives and funds in the futile hope of building an Afghanistan that can defend itself from Islamic terrorism. He is calling on allies such as India to support Afghan development.

North Korea’s ideology — known as “juche” — is as tenacious and murderous as the Islamist ideology. There is no evidence to suggest that we are attacking the Norks’ ideology. Why not?

I will not belabor what you faithful readers have read many times before in this column. Suffice it to say that unless the enemy’s ideology is defeated, the enemy is not substantially weakened far less defeated.

The third unlearned lesson is that we have to gather intelligence rapidly, from every source we can find and in every manner we can imagine. At home, we have shied away from gathering such intelligence for reasons of political correctness, the fear of offending anyone. The New York Police Department had a special unit gathering intelligence in Muslim gathering places, including mosques, and even in neighboring states. As soon as he was inaugurated, Mayor Bill de Blasio ended the unit’s operations.

The National Security Agency and the FBI use the Foreign Intelligence Surveillance Act to intercept communications between terrorists here and abroad. The relevant sections of FISA — including Section 702, which authorizes such snooping — will expire in December unless Congress acts to renew them. President Trump has yet to call for FISA’s reauthorization.

Why? Perhaps because he doesn’t believe in what the NSA and FBI are doing. Perhaps because his advisors haven’t properly briefed him on the essential nature of that sort of intelligence gathering, or perhaps he believes that because FISA’s authorities were abused by the Obama administration to apparently spy on his presidential campaign he thinks those authorities should be ended.

Note well that the denials of “wiretapping” Trump Towers were carefully phrased to limit them to that specific. There is no denial that Americans were targeted for interception of cell phone conversations or emails. If Mr. Trump had not tweeted about “wiretaps” on his phones but instead mentioned NSA and FBI interceptions of other types of communications, those denials could not have been made truthfully.

We are at war with an enemy that is bound to a religion and culture unlike ours. They do not share our values. We have never defeated any of them or their ideology. And — for the sake of political correctness and congressional cowardice — we are about to abandon some of the most essential tools of intelligence gathering we need in this war.

No wonder we’re losing.