Many of us – and as a Canadian I certainly include myself – have lived so long in what historians are starting to call the Long Peace that we have come to assume that war is an abnormality. Something that may afflict others, from different cultures in far-off places. War for us, we think, belongs firmly in the past. And it is true that large parts of the world have not had to endure state-to-state wars for decades. The majority of the world’s nations have also been spared the scourge of civil wars, although many have known violence from revolutionary insurrection. Stephen Pinker and others have also argued that we are conducting our internal affairs with greater civility and point to declining levels of homicide and physical assault around the globe. (His own country, the United States, is an outlier here with much higher murder rates than in Canada or Europe.)

In my BBC Reith Lectures, I am arguing that we should be careful not to assume that the peaceful parts of the world are particularly virtuous or that they represent a clear trend for humanity’s moving away from war. We have been fighting each other for a very long time – as far as we can tell, from the moment we started to organise ourselves and settle down as agriculturalists. And much of the world today is at war: in Afghanistan, Syria, Yemen, the great lakes district of Africa or Sudan. There are also the “frozen” conflicts so favoured by Vladimir Putin’s regime – in eastern Ukraine, for example – that at any moment could ignite wider struggles.

Much of today’s war is low level, fought with submachine guns, portable rockets, even machetes and hoes, but the great powers continue to prepare for advanced technological war on a massive scale. Moreover, war is making one of those technological leaps that it has made so often in the past, from bows and spears to gunpowder, or from horses and mules to the internal combustion engine. The current generation of fighter planes is probably the last that will have pilots. They will be replaced by computers with increasingly sophisticated artificial intelligence. And while in the 19th and 20th centuries war moved increasingly into new dimensions, whether below the sea or into the air, it is now moving into cyberspace.

The range of weapons at the disposal of military powers is terrifying in its capacity to damage the world and its inhabitants, perhaps even to bring humanity’s long story to its end. Nuclear proliferation has never entirely been brought under control and the arsenals of nuclear powers contain bombs far more powerful than those dropped on Hiroshima and Nagasaki. There are treaties governing the development of biological and chemical weapons, but they are only as effective as the will to enforce them. At the other extreme among weapons of mass destruction are drones and killer robots, which are cheap to make, easy to manipulate and often tiny but deadly.Yet many in the military and their civilian masters continue to think and plan as if war remains a feasible option. John Bolton, the US national security adviser, who seems to have the president’s ear for the time being, has talked about invading North Korea or Iran. Equally worrying, officials and opinion makers in the US and China talk with resignation – or perhaps anticipation – about how history shows that declining and rising powers are bound to fight each other. Once you accept that something is inevitable, you risk bringing it closer. History is not much help when it comes to predicting the future, but it can remind us of the warning signals that always come before wars – the heightened rhetoric, for example, or the inability to understand the other side. What both sides learned in the cold war, sometimes nearly too late, is that they needed to grasp how the other side was thinking and feeling and how it might read or misread signals. In 1983, the Soviet Union became convinced, wrongly, that the United States and its allies were planning a sneak nuclear attack in retaliation for the Soviet shooting down of the Korean airliner KAL 007. Luckily, the west realised this in time and called off a planned military exercise.

Stretcher bearers in Passchendaele, August 1917: ‘The tally of lives lost and resources wasted.’ Photograph: Universal History Archive/Un/REX

The past can show us how wars start, how rarely they turn out as planned and how difficult they can be to stop, much less end in ways that won’t provide fertilisation for future wars. Much has changed about war, but certain things remain constant.

Nations and the individuals who lead them fight out of greed, when they think they can wrest something – land, spoils or people – from another. Conversely, we fight to protect what we have and hold dear. Or wars can be about political ideology and religion, which can have many of the same features. Some of the most terrible wars we have seen have been fought in the name of making a perfect society. When you are creating utopia, existing lives are the price to pay for a future in which everyone is happy. Finally, wars are fought for the most basic of human emotions. Fear, for example, of what others might do. In 1914, the German high command felt the timing was good for war because by 1917, so they calculated, Germany would no longer be able to take on a rapidly strengthening Russia. Feelings about honour – maintaining it, defending it, showing it – have led to wars between countries, just as they do between gangs.

A poster in Weifang, China, shows one of the artificial islands it is building in the disputed South China Sea. Photograph: VCG/VCG via Getty Images

As history reminds us again and again, wars are not always made on the basis of rational calculations; often the contrary. Many commentators pointed out before 1914 that Europe risked a massive and costly stalemate if its powers went to war and that, in the end, no one would benefit. Four years later, that had been demonstrated in the tally of lives lost and resources wasted. War, as Prussian general Carl von Clausewitz said, has its own logic and once started cannot be easily stopped.

We will never agree on the causes – who, what, why – of the Great War, but we should remember that mistakes and incorrect assumptions played a key role in the final crisis in July. Austria-Hungary was determined to destroy Serbia and Germany gave its infamous blank cheque without properly thinking through the consequences. In Vienna and Berlin, they deluded themselves that Russia would not enter a war in defence of Serbia and that, if the war spread, Britain would not intervene to protect France. They could not predict, and we cannot predict today, how nations and their leaders will react in moments of extreme crisis, especially if public opinion is taken into account. If, say, American and Chinese vessels clash in the South China Sea, will those at home insist on standing strong?

So we in the west need to beware of complacency. We are as much a prey to violent emotions, to blundering into war, as the Spartans and Athenians once were. We need to remember war, not so we can draw from it lessons about how to use it and how to win, but to understand how easily it can happen and escape control and how hard it can be to end in a way that gives some basis for a lasting peace. We really do need to think about war if we want to avoid it.

Professor Margaret MacMillan’s Reith Lectures, The Mark of Cain, begin next Tuesday on BBC Radio 4