By WARREN KOZAK

For many Americans over the age of 55, Nov. 22 rarely passes without a wistful sense of sadness and the thought: What if? But today, 47 years after the assassination of John F. Kennedy, the initial feeling of shock and disbelief has long since been replaced by the sense that the world took a very bad turn on that day in 1963—one that we have never quite been able to correct.

An obscure tape clip has recently surfaced on YouTube that offers no better proof of this redirection. It’s almost as if a voice from our past has come back to guide us through the most serious national security threat we face today.

The date is Sept. 25, 1961. Kennedy is standing in front of the United Nations General Assembly. And we hear a president of the United States assert a direct and unapologetic definition of who we are as Americans as he offers a response to, of all things, terrorism. The tape clip has a grainy quality, but the words are timeless:

“Terror is not a new weapon,” the young president tells the world body. “Throughout history, it has been used by those who could not prevail either by persuasion or example. But inevitably, they fail either because men are not afraid to die for a life worth living, or because the terrorists themselves came to realize that free men cannot be frightened by threats and that aggression would meet its own response. And it is in the light of that history that every nation today should know; be he friend or foe, that the United States has both the will and the weapons to join free men in standing up to their responsibilities.”

“Free men standing up to their responsibilities”—there is a lasting quality in those seven words that harks back to who we are as a people, to our War of Independence and our frontier days. It describes 18-year-old Marines on Pacific islands and in Afghanistan today. And it carries even more weight because this president, 18 years earlier, had almost lost his own life as a Navy officer on a patrol torpedo boat, PT-109, during World War II. He served his country in war despite his privileged background because, like most men of his generation, he believed in freedom and standing up to aggression. In other words, he took responsibility.

“The United States has both the will and the weapons.” No apology for our strength. No apology for our past. No apology for who we are, and yet the world still admires him.

Because of the location of the speech—the U.N.—the tape clip seems to emerge from some parallel universe when one considers the bitter irony of everything that followed Kennedy at that same podium.

Only 13 years later, the General Assembly would treat Palestinian leader Yasser Arafat to a wild and sustained ovation as that chief inventor of modern terror compared himself to the men who founded America. Thanks to an alliance of the Soviet bloc, the nonaligned nations and the Arab world, Arafat was accorded the honors of a head of state on Nov. 13, 1974. This occurred a mere 25 months after the Olympic massacre in Munich by the Black September Palestinian terrorists, and barely five months after the Democratic Front for the Liberation of Palestine slaughtered 22 Jewish high-school students in Ma’alot, Israel.

Since that day in 1974, the General Assembly has watched a parade of dictators, thugs and killers, including, this fall, the Iranian who stole an election and has repeatedly threatened Israel with nuclear annihilation. They are welcomed to spout their bile.

Take a measure. On one hand, you have Yasser Arafat, Idi Amin, Mahmoud Ahmadinejad, Moammar Gadhafi, Hugo Chávez, Fidel Castro, Robert Mugabe. On the other hand, you have Kennedy. There is a terrible imbalance when one weighs the U.N.’s roster over the last half century.

The final terrible irony is the young president’s own murder in 1963 by a communist wannabe. His brother, Robert F. Kennedy, was assassinated five years later in the first act of Arab terrorism on U.S. soil. Palestinian-born Sirhan Sirhan shot him at close range after he won the hugely important California presidential primary in 1968. That act of terror subverted our electoral process and changed our history, all because of Robert Kennedy’s support of Israel.

Those words spoken by our 35th president are not part of the usual JFK repertoire—like his inaugural address or his Oval Office speeches. But they may be more significant because of where we find ourselves today. His message serves as a beacon—because we hunger for this clear and bold direction since our present leaders seem to lack the will that Kennedy projects. So we lean on our past to find the courage we require today: A president standing alone, a man meeting his responsibilities.

Mr. Kozak is the author of “LeMay: The Life and Wars of General Curtis LeMay” (Regnery, 2009).