If you could go back to 1889 and strangle Adolf Hitler in his crib, would you do it? At one level, the answer is obvious. Of course, you should.
If there had been no Hitler, presumably the Nazi Party would have lacked the charismatic leader it needed to rise to power. Presumably, there would have been no World War II, no Holocaust, no millions dead on the Eastern and Western fronts.
But, on the other hand, if there were no World War II, you wouldn't have had the infusion of women into the workforce.
You wouldn't have had the GI Bill and the rapid expansion of higher education in the United States. You wouldn't have had the pacification of Europe, Pax-Americana, which led to decades of peace and prosperity, or the end of the British and other empires.
History is an infinitely complex web of causations. To erase mistakes from the past is to obliterate your world now. You can't go back and know then what you know now. You can't step in the same river twice.
So it's really hard to give simple sound-bite answers about past mistakes. The question, would you go back and undo your errors, is unanswerable. It's only useful to ask, what wisdom have you learnt from your misjudgments that will help you, going forward?
Which brings us to Iraq. From the current vantage point, the decision to go to war was a clear misjudgment, made by then President George W. Bush and supported by 72 per cent of the American public who were polled at the time. I supported it, too.
What can be learnt?
The first obvious lesson is that we should look at intelligence products with a more sceptical eye. There's a fable going around now that the intelligence about Iraqi weapons of mass destruction was all cooked by political pressure, that there was a big political conspiracy to lie us into war.
That doesn't jibe with the facts. Anybody conversant with the Robb-Silberman report from 2005 knows that this was a case of human fallibility. This exhaustive, bipartisan commission found "a major intelligence failure": "The failure was not merely that the intelligence community's assessments were wrong. There were also serious shortcomings in the way these assessments were made and communicated to policymakers."
The Iraq war error reminds us of the need for epistemological modesty. We don't know much about the world, and much of our information is wrong.
A successful president has to make decisions while radiating hesitancy, staying open-minded in the face of new evidence, not falling into the traps that afflict those who possess excessive self-confidence.
The second lesson of Iraq concerns this question: How much can we really change other nations? Every foreign policy dilemma involves a calibration. Should we lean forward to try to influence this or that region? Or should we hang back, figuring we'll just end up making everything worse?
After the 1990s, many of us were leaning in the interventionist direction. We'd seen the fall of the apartheid regime, which made South Africa better. We'd seen the fall of communist regimes, which made the Eastern bloc nations better.
Many of us thought that by taking down Saddam Hussein, we could end another evil empire and gradually open up human development in Iraq and the Arab world.
Has that happened? In 2004, I would have said yes. In 2006, I would have said no. In 2015, I say yes and no, but mostly no.
The outcome in Iraq, so far, should remind us that we don't really know much about how other cultures will evolve.
We can exert only clumsy and indirect influence on how other nations govern themselves. When you take away basic order, people respond with sectarian savagery.
If the victory in the Cold War taught us to lean forward and be interventionist, the legacy of the 2003 Iraq decision should cause us to pull back from the excesses of that mentality, to have less faith in America's ability to understand other places and effect change.
These are all data points in a larger education - along with the surge and the recent withdrawals from Iraq and Afghanistan. I wind up in a place with less interventionist instincts than where Mr Bush was in 2003 but significantly more interventionist instincts than where President Barack Obama is inclined to be today.
Finally, Iraq teaches us to be suspicious of leaders who try to force revolutionary, transformational change.
It teaches us to have respect for trimmers, leaders who pay minute attention to context, who try to lead gradual but constant change. It teaches us to honour those who respect the unfathomable complexity of history and who are humble in the face of consequences to their actions that they cannot fully predict or understand.
NEW YORK TIMES