If we know one thing, we know wars are dreadful, the worst that can happen to mankind. Look at the massacres in today's Syria, the roadside bombs in Afghanistan, the child soldiers in Africa. Remember the terrible world wars of the last century.
So how can we deal with the awkward fact that, in the great sweep of human history, wars have surprisingly often produced the breakthroughs, in ideas as well as technologies, the rest of us then come to depend on?
And does that awkward fact tell us anything useful about today's world, and how we've changed?
Let's start with the awkward fact. You are reading this by using an alphabet. Alphabetic writing, symbols-for-sounds, was a brilliant invention. It is far easier and more flexible than using little pictures. It was invented by relatively obscure people in today's Syria and Lebanon.
But it didn't stay there.
These people, the Phoenicians, were forced out across the Mediterranean by the Assyrian war machine, which shook up the whole Middle East; and they spread this kind of writing round the ancient world.
Others picked up the Phoenicians' notion and developed it for their own ends. The Greeks had the good idea of separating vowels and consonants. The Hebrews had their version.
But thanks to the early spread of the alphabet, at a time when towns were being burned and slaves led off in chains, we have Homer, and the great old tales of the Greeks and the written-down Old Testament stories of the Hebrews.
It changed Western civilization. Chinese bureaucrats once had to learn around 400,000 different little pictures to be fully literate. Thanks to these people, we can start with a couple of dozen basic squiggles.
Without the Babylonian war machine, horrible as it was, we wouldn't have had this specific religious tradition that produced not just modern Judaism, but Christianity and Islam too.
Just the same happened in other parts of the world. Buddhism was the revelation of a minor princeling from Northern India, who went to search for the secret of how best to live, at a time when his country was wracked by endless civil wars.
The Chinese philosophy we call Confucianism happened after a civil servant, aghast at the increasingly violent and anarchic effects of Chinese wars, also went on the road.
The first great age of empires, beginning around three thousand years ago, was a time of ferocious violence. It featured swollen-headed butcher kings, charcoaled cities and flies buzzing on silent flesh - those Assyrians with uncouth names and a penchant for skinning their enemies alive; Babylonians deporting entire peoples as slave labour; Chinese emperors indulging in orgies of killing and Indian kings drenching cities in blood.
But it brought huge advances in technology, from metalworking, wheels, reliable gold and silver coins as currency, new ways of sailing and using camels and horses.
Back in Greece, the great age of the Athenian philosophers and democrats emerged after that city-state's heroic fight against the Persian invaders. Why did Greeks endlessly debate the best way to govern themselves? War. Their states were in constant competition, and threatened by the tyranny of the Persians.
Coins? One of the earlier Persian rulers, the great Cyrus, had earlier invaded the rich little kingdom of Lydia - on what is now the Turkish coast. Lydia happened to have a natural supply of gold and silver and produced the first reliable, properly-minted coins. Thus the idea of "currency" was spread round Persian Asia, and eventually the whole Mediterranean world.
War is terrible. But it makes things happen. It smashes different cultures into one another. The brief but enormous empire of Alexander the Great mingled Asia and Greece in a way which changed the course of history. The eruption of Genghis Khan's Mongol hordes shattered the rich Muslim cultures of central Asia, giving space for that once slightly obscure second-rate civilization, Christian Europe, to begin its glittering rise to dominance.
Empires need to be ruled, and that means bureaucrats, and mathematics and writing, and then good roads, or canals, and later on better shipping. Barbarian rulers arrive in a conquered capital, and learn things they had never dreamed of before - new religions, new ways of dressing and cleaning themselves. It's only thanks to the murderous Roman war machine that Britons first found about the pleasures of a good hot bath.
Reflecting on all this persuaded me to call one of the episodes of my BBC 1 world history, "The Case for War." It's a deliberately provocative title but, I think, very much worth arguing about. Of course, it is easier to defend the further back you go. We still have the inventions, and the changes, but we know very little about the people who died or were enslaved along the way.
And of course, there's the question about what would have happened had wars gone the other way. As something of a sucker for the "what-if?" or 'counter-factual" way of thinking about history, I often wonder what would have happened had the great general of Cathage, Hannibal, kept on marching south after his victory over the Romans, and destroyed Rome.
Would a classical world dominated by the seafaring, industrious Carthaginians have changed everything that followed?
Europeans might have made it to the Americas earlier. That would certainly have changed everything. When Christopher Columbus headed out across the Atlantic he thought he was heading for Japan. He couldn't do his sums and he had a confused idea of what he wanted to achieve.
But the arrival of Europeans in the Americas after such a long gap, when two parts of humanity had been separated for around 14,000 years, one side had horses and gunpowder for making war, and the other side didn't.
More importantly, the Americans had no resistance to European diseases and a population of perhaps 100 million people almost vanished - around 85%, maybe more, eventually died from disease. The effect on world history of a few armour-plated soldiers carrying primitive guns has been vast.
Overall, the course of "big history" - mankind's journey from nomad tribes to farmers and villagers, to town-dwellers and today's industrial, crowded world - would have happened anyway. But its rhythm and detail, its smell and its look, were often made by the accidents of war.
The big questions are, 'Is that still true today?' and 'Does that mean war is actually a good thing?' But the answers are 'no'.
The invention of the nuclear bomb, a casebook example of something that happened where and when it did because of the Second World War (ditto the jet engine, ditto the rocket) changed the rules. It meant that the dangers of all-out war between big countries now hugely overshadow any possible advantages.
Second, because we now live in a closely interconnected world where new inventions and ideas are passed from continent to continent almost instantly (think of the computer tablet or Twitter) there is no need for a war to spread new ideas and technologies.
Today's wars may have good effects or bad ones - they may remove a tyrant or put in place an intolerant theocracy - but they no longer advance general human progress.
Finally, some good news. The amount of violence among hunter-gatherers, and in early societies, was far greater than it is today. Murder was much commoner in pretty medieval villages than in the toughest modern housing estates. As we have been packed to live together, we have become less violent - you might even say, more civilized.
When we first emerged from Africa as a world-colonising ape, part of our secret was the tribe. We lived in groups that were larger than big families, but small enough to engender a strong sense of belonging. We could specialize, and help one another; but that relied on us being us, "the tribe" and therefore being at least wary and often hostile to other tribes.
That basic sense of "us and them" has been with us from the get-go. From football supporters to religious cults, great nations to tiny towns, it's inside us still.
So we've moved forward, we clever apes. We have become kinder and less violent - but our capacity, our instinct, for violence remains ticking away inside us all the time. The point about the Assyrians, or the early Greeks, is that the more you learn about them, the more familiar they seem. But in their time, warfare could produce wonderful unexpected benefits for humanity. Now it produces... well, orphans, mostly.
Andrew Marr's History of the World starts 23 September, 9pm, BBC One
The book, A History of the World is published by Macmillan on 27
September, £25.00Suggest a correction