It is one of the great mysteries of modern political life that our leaders seem so reluctant to learn from their mistakes.
You would have thought, after the dismal experience of the recent past, that they would have learned by now that when you put the words "military intervention" in the same sentence as "regime change", bad things tend to happen.
Today of all days, the anniversary of the 9/11 attacks, let's look at the record:
Afghanistan, 2001: A US-led alliance overthrows the Taliban following 9/11; 14 years later, after countless deaths, the country is still in deep trouble and the Taliban are still a force to be reckoned with.
Iraq, 2003: A US-led alliance overthrows Saddam Hussein; 12 years later, after countless deaths, the country is still in chaos and at risk of permanent fracture.
Libya, 2011: a NATO-led alliance helps local forces overthrow Muammar Gaddafi; four years later, after countless deaths, the country is in chaos with no effective government and has become one of the main embarkation points for refugees seeking sanctuary in Europe.
So now, when I hear David Cameron say he is again contemplating asking parliament for authorisation to take military action in Syria - and foreign secretary Philip Hammond says that President Assad "cannot be part of Syria's future" - my heart sinks.
I also wonder why they think that striking at the "controlling brains" of IS is likely to be a good move, given that doing the same thing against the leaders of jihadi groups in Pakistan, Afghanistan, Iraq and Yemen seems not to have been spectacularly successful. Old thinking, someone once said, equals lazy thinking.
But I mustn't be unfair, because there are, in fact, some signs that a few lessons may have been learned. At least Mr Hammond told MPs: "We are not saying Assad and all his cronies have to go on day one." And it seems there is significant behind-the-scenes diplomatic activity aimed at drawing up a multi-national Syria strategy involving both Assad's main sponsors, Russia and Iran.
Bashar al-Assad is responsible for some of the most heinous war crimes of recent times, including the use of chemical weapons, the mass imprisonment and torture of political opponents, and the indiscriminate bombing of civilian areas causing massive casualties. Yet the unpalatable reality must surely be that, despite his grim record, he remains indispensable in the search for an end to the conflict.
The Western powers, Russia and Iran share with him a common enemy: the blood-thirsty zealots of the Islamic State group. It is in everyone's interests to limit IS's capacity to spread suffering across Syria and Iraq, and, allegedly, to plan attacks against Western targets well away from the Middle East.
It shouldn't be beyond the wit of the world's best diplomats to find a formula for a peace process that satisfies both Assad's backers and the Saudis and Gulf states who want to halt what they see as Tehran-sponsored Shia expansionism across the region. The risk is that the Saudi-Iranian, Sunni-Shia battle for regional hegemony, of which Syria is a part, develops into a semi-permanent succession of proxy wars, much as the capitalist-communist Cold War did between 1945 and 1989.
And while we're on the subject of Cold Wars, it's worth noting the rising concern in Washington and elsewhere about Russian military moves in Syria. According to a report in Foreign Policy: "US officials are concerned that a dramatic Russian military buildup in western Syria over the past week signals preparations by Moscow to fly combat missions in support of President Bashar al-Assad's regime ... At least four Russian Condor cargo planes and several naval ships have delivered an array of military equipment and hardware in recent days at an airfield near Latakia on the Mediterranean coast and at the Russian naval facility of Tartus."
Washington and London are right that Syria has no chance of recovery as long as President Assad remains in power. But now is not the time to say so. We do not need to ignore his responsibility for the agonies that Syria is suffering (according to one estimate, 968 of the 1,205 civilians killed last month were killed by Assad's forces) to recognise that he is also part of the solution.
When Churchill confronted the Nazis in World War Two, he was prepared to deal with Stalin as an ally, even though the Soviet leader was one of the worst mass murderers of the 20th Century. Similarly, the US president Richard Nixon was prepared to negotiate with the Chinese Communist leader Mao Zedong, even though Mao, like Stalin, was responsible for the deaths of many millions of his own people.
Assad is neither Stalin nor Mao, and IS are not Nazis. The point is simply that both diplomacy and war often involve alliances with unpleasant people. The guiding principle is the greater good - and for now, the priority in Syria must be to reduce the level of violence and halt the spread of IS.
It is in the interests of the people of Syria, and in the interests of all those countries struggling to cope with the tens of thousands of refugees fleeing from the war. Half the country's population of 22 million have already fled from their homes; four million of them have sought sanctuary in Lebanon, Jordan and Turkey.
The truth is that they will not stop fleeing until the war is over. And all the most recent evidence - from Afghanistan, Iraq, Libya and Yemen - strongly suggests that you do not stop wars by dropping more bombs.