Since early November, scrappy debates have raged over the relative importance of the three catastrophes the United States has endured in the past three-quarters of a century. Which one changed America the most? The Japanese attack on Pearl Harbor in 1941, the assassination of President John F. Kennedy in 1963, or the terrorist attacks on New York City and the Pentagon in 2001?

My parents, lefties and internationalists, had been hoping the United States would get into the war Germany had begun in 1939 by invading Poland. Like President Franklin Roosevelt, they believed U.S. interests lay in helping beleaguered Britain, rescuing defeated France, and opening a second front somewhere to relieve the Nazi assault on the Soviet Union.

Isolationist sentiment, however, was strong throughout the country. For many, the lesson of World War I was not to meddle in European affairs again. No one, even in California where I was growing up, thought much about Japan. I recall exactly where I was on Dec. 7, 1941, because it was a special treat.

We lived 60 miles east of Hollywood, and my parents wrote screenplays. Under their deal with Twentieth Century Fox, they were permitted to write at home and come into the studio for conferences with their producer. But on Sunday, Dec. 7, even though it was wintry in the Midwest and the East, it was balmy enough in Southern California for a softball game between the Fox producers and the writers.

Essentially, the producers patronized and bullied the writers, and the writers hated the producers. My father, a magnificent athlete who starred as a running back in college, had just hit a triple and was standing on third base when, abruptly, the game was halted.

Little war monger that I was, I desperately wanted my father to enlist and go fight. No, my mother said, he was in the last one so he doesn’t go to this one.

Twenty-two years later, the assassination of John F. Kennedy cut short a promising presidency, opening the door to almost unlimited escalation in Vietnam. Kennedy had begun the intervention himself, but when he died there were only 16,000 American troops in Vietnam.

His death was a blow to the bounding hopes of an entire generation. My filmmaking boss, a Republican who happened to be exactly Kennedy’s age, closed the door to his office for the only time in the four years I worked for him. He opened it 10 minutes later and, red-eyed, told us all to go home.

“We will never laugh again,” the Washington columnist Mary McGrory said to Daniel Patrick Moynihan, a future senator who worked in the Kennedy administration.

“We’ll laugh again,” Moynihan told her, “but we will never be young again.”

The first plane hitting the World Trade Center was described as an accident on the morning of Sept. 11, 2001. I was driving from Bangor to Castine listening to the radio when the crash of the second plane into the South Tower was reported, clarifying the flight plan of both planes as anything but accidental.

This led to two more American wars, a loss of blood and treasure in the Middle East almost beyond tabulation, and the conflagration — military, political and emotional — of an entire region that has yet to be extinguished. If anything, every year since 2001, and sometimes it seems every month, the fire begun on Sept. 11 burns ever more brightly.

In one sense, the debates over which tragedy changed us the most — carried on over the Internet, in bars, in classrooms — are as fruitless as arguments over the course history may have taken if Christopher Columbus had actually reached the East Indies as he thought he had.

But in another sense, what-ifs are not an altogether wasted intellectual calisthenic when they help us think about, and perhaps even avoid, history’s detours that await us. If we turn around the three catastrophes and make informed guesses about what would have happened had they not occurred, we may learn something.

If there had been no 9/11 attacks, the lesson of Vietnam could have stayed in our national consciousness. For a whole generation after our disaster in Vietnam, we stayed out of wars against countries whose leaders annoyed but didn’t threaten us. Suddenly, 9/11 produced — like a hard blow to the head — amnesia in the body politic. We forgot we’d been lied into the war in Vietnam. We forgot that, far from being members of a Communist monolith, Vietnam and China hated each other. We forgot we went into Vietnam without understanding its history, religion or culture.

So we did it again.

Afghanistan, at best dysfunctional, at worst a failed nation in the midst of a civil war, had to be invaded. A year and a half later, we allowed ourselves to be led, and lied, into war against Iraq, which had neither weapons of mass destruction nor an alliance between the irreligious Saddam Hussein and the religious extremist Osama bin Laden, both of which fantasies had been peddled by the Bush administration. Surely bin Laden died happy for having sucked and suckered us into wars against two Muslim countries and political conflicts with several others.

Without 9/11, no wars would have been fought against Muslims, begun mendaciously by President George W. Bush, continued erroneously and even expanded in Afghanistan by President Barack Obama.

The phenomenon of John F. Kennedy, if it demonstrates nothing else, is vivid proof that an individual life, and death, can make an immense difference. If he had lived he would most likely have won the 1964 election against Barry Goldwater handily, though not as overwhelmingly as Lyndon Johnson did.

Kennedy had the talent and leadership qualities to have become a great president in a second term. From the crisis he went through with the Soviet Union over Cuba he had learned how to defuse potential explosions before they detonate. It’s very hard to believe he ever would have put half a million troops into Vietnam as his successor did. He didn’t deal with Congress as expertly as Johnson, but he cared intensely about civil rights as he did about inequality in general. He would have pushed hard on both fronts. In foreign affairs he had learned after a failed early meeting with Soviet leader Nikita Khrushchev how to deal with his adversaries. Johnson had no real admirers in Europe; after his “Ich bin ein Berliner” speech at the Berlin Wall, Kennedy had legions.

After World War I the United States retreated into isolationism. As Nazi Germany began swallowing European countries, FDR and many of his supporters wanted the United States to act against Hitler. But the prevailing American sentiment was to stay out of war. The Japanese attack on Pearl Harbor changed all that. America would never be isolationist again. The country was now on the world stage for keeps.

If the Japanese had not attacked Pearl Harbor, the United States would not have had to fight a two-front war. Historians agree that sometime in 1942 FDR would have found a way to confront the German domination of Europe in which the Nazis had already beaten our oldest ally, France, and grievously damaged our best friend, Great Britain.

Germany would surely have been defeated before 1945. The gathering of Nazi officials, known to history as the Wannsee Conference, would still have occurred in 1942. These insane bureaucrats would still have decided on their “final solution to the Jewish question” and determined that the Jews of Europe were to be murdered. But the extermination would have been far less complete.

To cite merely one example out of 6 million, one innocent teenager whose name and work continue to inspire and move us, Anne Frank did not die at the Bergen Belsen concentration camp until March 1945.

Well before that Germany would have surrendered had we not also been fighting Japan. American might that was expended in the Pacific at the battles of Midway, Guadalcanal, Okinawa, Coral Sea, Saipan, Iwo Jima, and Leyte Gulf would have been concentrated on North Africa, in the Mediterranean, the Atlantic and the heart of Europe much earlier. There would have been no resort to atomic bombs because Germany could not have held out nearly as long as Japan did.

If the top Japanese admiral, Isoroku Yamamoto, who warned against war with the United States, had been heeded, Japan’s leaders might have contented themselves with their growing empire in the east. It was Yamamoto who was reported to have said after Pearl Harbor, “I fear all we have done is to awaken a sleeping giant.” Without Pearl Harbor, a militarized Japan would have remained a bulwark against the Chinese expansionism we see today.

In which circle of American hell do we choose to dwell? Which of the three disasters most changed our history? No one can ever prove whose speculation is superior, but my vote goes to Pearl.

Peter Davis is a writer and filmmaker who lives in Castine. He covered the war in Iraq for The Nation magazine and received an Academy Award for his film on the Vietnam War, “Hearts and Minds.”