As the literal meaning of the word indicates, terror is a military strategy that hopes to change the political situation by spreading fear rather than by causing material damage. This strategy is almost always adopted by very weak parties, who are unable to inflict much material damage on their enemies. Of course, every military action spreads fear. But in conventional warfare, fear is a byproduct of material losses, and is usually proportional to the force inflicting the losses. In terrorism, fear is the whole story, and there is an astounding disproportion between the actual strength of the terrorists and the fear they manage to inspire.

It is not easy to change the political situation through violence. On the first day of the battle of the Somme, 1 July 1916, 19,000 members of the British army were killed and another 40,000 wounded. By the time the battle ended in November, both sides together had suffered more than a million casualties, which included 300,000 dead. Yet this unimaginable carnage hardly changed the political balance of power in Europe. It took another two years and millions of additional casualties for something finally to snap.

Compared to the Somme offensive, terrorism is a puny matter. Most terrorist attacks kill only a handful of people. In 2002, at the height of the Palestinian terror campaign against Israel, when buses and restaurants were hit every few days, the yearly toll reached 451 dead Israelis. In the same year, 542 Israelis were killed in car accidents. A few terrorist attacks, such as the bombing of Pan Am flight 103 over Lockerbie in 1988, kill hundreds. The 9/11 attacks set a new record, killing nearly 3,000 people. Yet even this is dwarfed by conventional warfare: if you add all the people killed and wounded in Europe by terrorist attacks since 1945 – including victims of nationalist, religious, leftist and rightist groups – it will still represent many fewer casualties than in any number of obscure first world war battles, such as the third battle of the Aisne (250,000 casualties) or the 10th battle of the Isonzo (225,000 casualties).

A fragment of the cockpit of the Pan Am Boeing 747 that exploded over the village of Lockerbie on 21 December 1988. Photograph: Letkey/AFP

How then can terrorists hope to achieve much? Following an act of terrorism, the enemy continues to have the same number of soldiers, tanks and ships as before. The enemy’s communication network, roads and railways are largely intact. Its factories, ports and bases are hardly touched. However, the terrorists hope that even though they can barely dent the enemy’s material, power, fear and confusion will cause the enemy to misuse its strength. Terrorists fight like tai-chi masters: they aim to beat the rival with the rival’s own power. Thus the French in Algeria in the 1950s were not defeated by the FLN, the National Liberation Front, but by their misguided reaction to FLN terrorism. The American debacles in Iraq and Afghanistan were the result of the Americans misusing their immense power, rather than of al‑Qaida flexing its miniature muscles.

Terrorists calculate that when the enraged enemy uses its massive power against them, it will raise a much more violent military and political storm than the terrorists themselves could ever create. During every storm, many unforeseen things happen. Mistakes are made, atrocities are committed, public opinion wavers, questions are asked, neutrals change their stance and the balance of power shifts. The terrorists cannot foretell what the result will be, but they have a much better chance fishing in such troubled waters than when the political sea is calm.

Reshuffling the cards

Terrorism is a very unattractive military strategy, because it leaves all the important decisions in the hands of the enemy. Since terrorists cannot inflict serious material damage, all the options the enemy had prior to a terrorist attack are at its disposal afterwards as well, and it is free to choose among them. Armies normally try to avoid such a situation at all costs. When they attack, they seek not to provoke the enemy to action, but rather to reduce the enemy’s power to retaliate, and in particular, to eliminate its most dangerous weapons and options. For example, when the Japanese attacked the US Pacific fleet at Pearl Harbor in December 1941, they could be certain about one thing: no matter what the Americans decided to do, they would not be able to send a fleet to south-east Asia in 1942.

Provoking the enemy to action without eliminating any of its weapons or options is an act of desperation, taken only when there is no other way. Whenever it is possible to inflict serious material damage, nobody gives that up in favour of mere terrorism. It would have been madness if, in December 1941, the Japanese had torpedoed a civilian passenger ship in order to provoke the US but left the Pacific fleet in Pearl Harbor intact.

People turn to terrorism because they know they cannot wage war, so they opt instead to produce a theatrical spectacle. Terrorists don’t think like army generals; they think like theatre producers. The public memory of the 9/11 attacks is evidence of that: if you ask people what happened on 11 September 2001, they are likely to say that al-Qaida destroyed the twin towers of the World Trade Center. Yet the attack involved not merely the towers, but two other actions, in particular a successful attack on the Pentagon. Why is it that only a few people take proper notice of that? If the 9/11 operation had been a conventional military campaign, the Pentagon attack would have received most of the attention. In this attack, al‑Qaida managed to destroy part of the enemy’s central headquarters, killing and wounding senior commanders and analysts. Why is it that public memory gives far more importance to the destruction of two civilian buildings, and the killing of brokers and accountants?

There is an astounding disproportion between the actual strength of the terrorists and the fear they manage to inspire

It is because the Pentagon is a relatively flat and unassuming building, whereas the World Trade Center was a tall, phallic totem whose collapse created an immense audiovisual effect. Nobody who saw the images of its collapse will ever forget them. We intuitively understand that terrorism is theatre, and hence we judge it by its emotional rather than material impact. With hindsight, Osama bin Laden would probably have preferred to launch the plane that hit the Pentagon against a more picturesque target, such as the Statue of Liberty. True, few people would have been killed and no military assets would have been destroyed, but just think what a powerful theatrical gesture it would have been.

Like terrorists, those combating terrorism should also think more like theatre producers and less like army generals. Above all, if we want to fight terrorism effectively we must realise that nothing the terrorists do can defeat us. We are the only ones who can defeat ourselves, if we overreact in a misguided way to terrorist provocations.

The rubble of the World Trade Center in New York following the 11 September 2001 attack Photograph: ALEX FUCHS/AFP

Terrorists undertake an impossible mission: to change the political balance of power when they have almost no military abilities. To achieve their aim, they present the state with an impossible challenge of its own: to prove that it can protect all its citizens from political violence, anywhere, anytime. The terrorists hope that when the state tries to fulfil this impossible mission, it will reshuffle the political cards, and hand them some unforeseen ace.

True, when the state rises to the challenge of protecting its citizens from political violence, it often succeeds in crushing the terrorists. Hundreds of terrorist organisations have been wiped out over the last few decades by various states. In 2002-04, Israel proved that a terror campaign can be suppressed by brute force. Terrorists know full well that the chances in such a confrontation are against them. But since they are very weak, and have no other military option, they have nothing to lose and much to gain. Once in a while the political storm created by counterterrorist campaigns does benefit the terrorists, which is why the gamble makes sense. A terrorist is akin to a gambler holding a particularly bad hand, who tries to convince his rivals to reshuffle the cards. He cannot lose anything, but he may win everything.

A small coin in a big empty jar

Why should the state agree to reshuffle the cards? Since the material damage caused by terrorism is negligible, states could theoretically do nothing about it, or take strong but discreet measures far from the cameras and microphones. Indeed, they often do exactly that. But every now and then states lose their tempers, and react far too forcefully and publicly, thus playing into the hands of the terrorists.

Why are they so sensitive to terrorist provocations? Because the legitimacy of the modern state is based on its promise to keep the public sphere free of political violence. A regime can withstand terrible catastrophes, and even ignore them, provided its legitimacy is not based on preventing them. On the other hand, a regime may collapse due to a minor problem if it is seen as undermining its legitimacy. In the 14th century the Black Death killed between a quarter and a half of European populations, yet no king lost his throne as a result, even though no king made much of an effort to overcome the plague. Nobody back then thought that preventing plagues was part of a king’s job. On the other hand, rulers that allowed religious heresy to spread in their dominions risked losing their crown, and even their head.

Today, a government may turn a blind eye to high levels of domestic and sexual violence, because they do not undermine its legitimacy. In France, for example, more than 1,000 rape cases are reported to the authorities each year, with thousands more unreported cases. Rapists and abusive husbands, however, are not perceived as an existential threat to the state, because, historically, the state did not build itself on the promise to eliminate sexual violence. In contrast, the much rarer cases of terrorism are viewed as a deadly threat, because over the last few centuries modern western states have gradually built their legitimacy on the explicit promise to maintain zero political violence within their borders.

Back in the middle ages, the public sphere was full of political violence. Indeed, the ability to use violence was the entry ticket to the political game, and whoever lacked this ability had no political voice. Not only did numerous noble families retain armed forces, but so did towns, guilds, churches and monasteries. When an abbot died and a dispute arose about the succession, the rival factions – comprising monks, local strongmen and concerned neighbours – often used armed force to decide the issue.

Terrorism had no place in such a world. Anybody who was not strong enough to cause substantial material damage was of no consequence. If in 1150 a few Muslim extremists had murdered a handful of civilians in Jerusalem, demanding that the Crusaders leave the Holy Land, the reaction would have been ridicule rather than terror. If you wanted to be taken seriously, you should have at least gained control of a fortified castle or two. Terrorism did not bother our medieval ancestors because they had bigger problems to deal with.

During the modern era, centralised states have gradually reduced the level of political violence within their territories, and in the last few decades western countries have managed to achieve almost zero political violence. Their citizens can struggle for control of towns, corporations, organisations and even of the government itself without any need of brute force. Command of hundreds of billions of euros, hundreds of thousands of soldiers, and hundreds of ships, aeroplanes and nuclear missiles passes from one group of politicians to another without a single shot being fired. People have quickly got used to this, and consider it their natural right. Consequently, even sporadic acts of political violence that kill a few dozen people are seen as a deadly threat to the legitimacy and even survival of the state. A small coin in a big empty jar can make a lot of noise.

This is what makes the theatre of terrorism so successful. The state has created a huge space empty of political violence. This huge space acts as a sounding board, amplifying the impact of any armed attack, however small. The less political violence in a particular state, the greater the public shock at an act of terrorism. Killing 17 people in Paris draws far more attention than killing hundreds in Nigeria or Iraq. Paradoxically, then, the very success of modern states in preventing political violence make them particularly vulnerable to terrorism. An act of terror that would have gone unnoticed in a medieval kingdom can rattle much stronger modern states to their very core.

The state has stressed so many times that it will not tolerate political violence within its borders that it has no alternative but to see any act of terrorism as intolerable. The citizens, for their part, have become used to zero political violence, so the theatre of terror incites in them visceral fears of anarchy, making them feel as if the social order is about to collapse. After centuries of bloody struggles, we have crawled out of the black hole of violence, but we feel that the black hole is still there, patiently waiting to swallow us again. A few gruesome atrocities and we imagine that we are falling back in.

In order to assuage these fears, the state is driven to respond with its own theatre of security. The most efficient answer to terrorism might be good intelligence and clandestine action against the networks of money that feed terrorism. But this is not something citizens can see on television. Once its citizens have seen the terrorist drama of the World Trade Center collapsing, the state feels compelled to stage an equally spectacular counterdrama, with even more fire and smoke. So instead of acting quietly and efficiently, it unleashes a mighty storm, which fulfils the terrorists’ most cherished dreams.

Terrorism goes nuclear

Yet what about nuclear terrorism or bioterrorism? What if the doomsday prophets are correct, and terrorist organisations will acquire weapons of mass destruction, which could cause widespread material damage on a par with conventional warfare? If and when this comes to pass, the state as we know it will become outdated. However, terrorism as we know it will also cease to exist, like a parasite that dies along with its host.

If tiny organisations representing a handful of fanatics could destroy entire cities and kill millions, there will no longer be a public sphere free of political violence. Politics and society will undergo radical transformations. It is hard to know how political struggles will be conducted then, but they will certainly be very different from the terror and counterterror campaigns of the early 21st century. If in 2050 the world is full of nuclear and bio-terrorists, their victims will look back at the western world of today with longing tinged with disbelief: how could people who lived such secure lives nevertheless have felt so threatened?

• Yuval Noah Harari is the author of Sapiens: A Brief History of Humankind.