A number of years ago, when I was dating the woman who became my wife, we were invited to dinner at the home of close friends. There was another couple there that night that had just moved to New York from Washington, where they both worked in the Clinton White House — he for Bill, she for Hillary. Throughout the evening, they regaled us with fascinating stories about the first couple that were both fairly personal and completely unsolicited. Just before dessert, I turned to my future wife and whispered, “I am now convinced, more then ever, that Lee Harvey Oswald acted alone.”

She shot me a look that I have since grown accustomed to that implies “I’m seriously concerned about your thought process.” As soon as we were on the street, she asked what I meant. If I’ve learned anything, I explained, the one thing I am sure of is that people will always talk and people in Washington talk more than most. It’s usually out of ego, less often from guilt. Now imagine a conspiracy on the magnitude of the Kennedy assassination that has not leaked for 43 years involving, according to conspiracy theorists: the CIA, Lyndon Johnson, the mafia, the Cubans, the Russians, the extreme right wing, the extreme left wing, and Southern segregationists. It defies human nature — and logic. But logic is completely irrelevant when it comes to conspiracy thinking.

So it should come as no surprise that five years after 9/11, there now appears to be a growing number of people who say Al Qaeda did not perpetrate the attacks on that day. They believe this, in spite of the fact that the terror was witnessed firsthand by thousands of people and tens of millions more on live television. Osama bin Laden has even taken credit for the attacks, and that confession is captured on videotape as well. But to conspiracy theorists, evidence is as irrelevant as truth. Every man, woman, and child on earth could have been standing on Chambers Street on the morning of September 11, 2001. It doesn’t matter.

The theories are pretty far-fetched, but that doesn’t matter either — the more outrageous and illogical they are, the more legitimate they become. So a professor at the University of Wisconsin (no less) says our vice president, Dick Cheney, was behind it all (and the professor keeps his job). Charlie Sheen, that eminent actor, tells us it wasn’t the jets but a controlled explosion inside the towers that caused the World Trade Center to fall. The one I find particularly off the charts: the passengers on the four doomed jets were transferred to military planes at an airport in the Midwest and then dropped into the Atlantic Ocean. The original lie began in the Islamic world (steeped in a tradition of intrigue and lies) as the smoke was still rising over Manhattan. It claimed that all Jews were warned to stay away that day because Israel was really behind it all — a lie that has become rock-solid belief in the Arab world today. Now, following the terror and blood they sent us five years earlier, their lies have come to our shores as well, adding insult to injury.

Official and costly investigations have no impact on conspiracy theorists whatsoever. We could have saved the time and money. The 9/11 Commission, just like the Warren Commission before it, is considered illegitimate by the theorists because of the very fact that it was commissioned by the government. Investigations don’t matter because conspiracy theorists make their theories fit their own agendas. As improbable as their theories may seem to the rational mind, it’s important to remember that they still believe what they are saying. The people who subscribe to these theories have a strong need to feel superior to those around them, the flip side of which is that they actually suffer from feeling rather inferior.

Ironically, Lee Harvey Oswald, the poster child for an inferior personality, probably would have been a first-class conspiracy theorist himself. He succeeded at almost nothing in his pathetic life except, unfortunately, his final act. His colossal stupidity led him to the Soviet Union, where he envisioned he would be hailed a hero. When that didn’t happen, he whined to the American embassy that he wanted to return home. His marriage was not a success. He worked at a boring, minimum-wage job he thought was beneath him.

So it is understandable why so many Americans have a hard time grasping how this nothing of an individual could have had such an enormous impact on our history. Ultimately, that’s precisely the trump card that assassins, and terrorists, hold. They change history. Their prior résumés are as irrelevant as they are unimpressive. All the rest of us have the harder part of this bargain. We have to accept the reality of these events, even though large national tragedies are, by definition, difficult to accept. And there will always be people who just can’t do that — who have the need to believe that Lyndon Johnson was behind Kennedy’s death or that Franklin Roosevelt orchestrated Pearl Harbor or that our vice president engineered 9/11 to get us into Iraq or, on a somewhat different note, that Elvis never really died. There is something else that helps lead susceptible minds to conspiracy: the shocking incompetence of government agencies in these events. At Pearl Harbor, the Navy demonstrated an appalling lack of foresight. On September 11, it was the astonishing failure of our intelligence services. In 1963, the events were compounded by the monumental ineptness of the Dallas Police Department, the FBI, and, ultimately, the Secret Service. “Explanations seemed pitifully inadequate. Even after the facts were in, dregs of doubt remained. Conspiracy seemed to be a reasonable deduction,” wrote the historian William Manchester.

Something profound happened to America on November 22, 1963, just as it happened on September 11, 2001, and on December 7, 1941, that was inexplicable. In the first case, a young, vibrant president was gunned down in broad daylight in an American city. In 1963, Americans naively believed this sort of thing just didn’t happen in their country. Sure, three presidents had been assassinated before, but the last one was McKinley in 1901. Similarly, 38 years later, when we watched plane after plane crash into our landmarks on “Good Morning America” and then saw two of the largest and most remarkable buildings in the world come tumbling down with a horrifying loss of life, it defied any previous part of our national consciousness.

On December 7, 1941, Americans also gaped at the news that their entire Pacific Fleet had been destroyed by what they assumed was a highly inferior military power. But here is the one big difference: On December 8, Americans didn’t sink into la-la land. They got serious about the realities of the event. The infighting vanished. The conspiracy theorists were probably there, but they couldn’t find a microphone to shout their insanity. Everyone else was too busy dealing with the danger at hand to waste their time with such nonsense.

Mr. Kozak is a regular contributor of The New York Sun.