Although this was not the first hijacking of a plane - many had been temporarily diverted from their intended destinations in order to get the hijackers somewhere else, generally Cuba - it was the first one designed to terrorise a nation into changing its policy and to force world attention on a cause. The hijacking was the first in a series of dramatic and well-coordinated attacks planned and approved at the highest levels of the PLO. The purpose of these hijackings was subsequently acknowledged, indeed proclaimed, by the PLO's chief observer to the UN: to bring public attention to the Palestinian cause as an initial step in a multifaceted programme of terror, the goal of which was the establishment of a Palestinian state.

Success begets repetition. By the early 70s, the pattern was becoming quite discernible. Terrorists who hijacked, blew up or otherwise attacked commercial airliners would, if captured, quickly be released by most countries. Only Israel would keep captured terrorists in prison for substantial terms. The message was clear. Terrorist attacks committed outside Israel would go unpunished and would generally achieve the desired result.

It was against this background that Palestinian terrorist organisations planned the most dramatic act of international terrorism up until that time: the 1972 attack on the Israeli Olympic team in Munich. Based on the reaction to international terrorism over the previous four years, the terrorists planning the Munich operation could expect to succeed in attracting the world's attention and be relatively certain that if any of the terrorists were captured they would not be held for long. They could expect to be treated as heroes by supporters of their cause, and as "official state guests" by the European governments that held them for a brief period before sending them home.

But international reaction went beyond merely "taking note" of the Palestinian cause. Even decent people cannot ignore indecent events. It is only natural to talk about evil. What many in the international community did was to ascribe a positive moral content to the evils of Palestinian terrorism. The assumption seemed to be that any group of people willing to resort to such extreme measures must have a just and compelling cause.

Soon afterwards, the UN granted observer status to the PLO - the first terrorist organisation to be so honoured. Recall that all this was taking place while the PLO was still boasting of its reliance on terrorism as its primary mechanism of "diplomacy". Bruce Hoffman, the Rand Corporation's director of terrorism research, saw a likely causal relation between the willingness of the Palestinians to resort to terrorism and their acceptance by the UN: "It is perhaps not entirely coincidental, then, that 18 months after Munich the PLO's leader, Yasser Arafat, was invited to address the UN general assembly and shortly afterwards the PLO was granted special observer status in that international body. Indeed, by the end of the 70s the PLO, a non-state actor, had formal diplomatic relations with more countries (86) than Israel (72). It is doubtful whether the PLO could have achieved this success had it not resorted to international terrorism."

If anyone actually believed that honouring Arafat and legitimising his terrorist organisation would persuade them - or anyone else - to abandon terror, they were quickly proved wrong. Within 10 days of Arafat's speeches Palestinian terrorists hijacked a British airliner in Dubai. The plane was eventually flown to Tunisia, where a passenger was killed. Over the half decade following the legitimisation of Arafat and his cause, Palestinian terrorists attacked Orly airport, hijacked a plane to Entebbe, Uganda (where Israeli commandos rescued all but three passengers), attacked a passenger terminal at Istanbul, hijacked a Lufthansa plane, hijacked an Israeli bus, killing 26 civilians, attacked an El Al plane in Paris, and shot passengers in Brussels. During this same half decade, the UN declared its first international day of solidarity with the Palestinian people. Palestinian terrorism was working.

The European policy of appeasing terrorists and encouraging their acts continued unabated through the 80s, as did Palestinian terrorism. Perhaps the most publicised instance of this European "policy" of appeasing terrorists came on the heels of the hijacking of an Italian cruise ship, the Achille Lauro. In October 1985, armed PFLP terrorists boarded the ship as it left Alexandria, Egypt. In one of the most cowardly acts in the history of terrorism, they murdered a Jewish-American man in a wheelchair. Once again American plans for a rescue were thwarted by the Italian government, whose "foreign policy required it to maintain very close relations with the Arab states and the PLO", according to Philip Heymann, the former US deputy attorney-general under President Clinton and author of Terrorism and America.

The terrorists were freed, with the approval of the Italian prime minister Bettino Craxi, who "later said somewhat implausibly [that he did not know] that they had murdered a wheelchair-bound American, Leon Klinghoffer," Heymann wrote. But even that absurd excuse was not available to Craxi when he once again frustrated efforts by the US to bring the terrorists to justice. After the hijackers were freed in Egypt, a small Egyptian plane was flying them to a hero's welcome in another friendly Arab country. The US military intercepted the plane in midair and forced it to land at a Nato base in Italy, where American C-141 troop transporters were waiting to fly the terrorists to the US to stand trial for the hijacking of the ship and the murder of Klinghoffer. But Italian soldiers blocked the Americans from approaching the Egyptian plane. Finally, after the Italians promised to bring the hijackers to justice, the US backed down and the four hijackers were taken into Italian custody. Among them were two important terrorist leaders with rivers of blood on their hands. The Italians released them immediately. The others were tried, convicted and allowed to " 'escape' while on leave from Italian prisons", in Heymann's words. As the Palestinian reliance on terrorism increased, most of the international community became more - not less - supportive of the Palestinian cause.

The message being sent by the international community was unmistakable: we will not punish you or your cause for your use of terrorism; to the contrary, we will reward you, honour your leaders and further your cause. If this was the message - and how can any reasonable observer deny that it was? - then what did the international community expect the terrorists to do? If a tactic is working, why change it, especially if the international community seems to approve of it?

The international community responded to terrorism between 1968 and 2001 by rewarding and legitimising it, rather than punishing and condemning it. Seen in this light, it is no wonder we had to suffer the horrors of September 11, 2001. Those who bestowed these benefits on the Palestinians following their terrorism, especially our European allies and the UN, made September 11 unavoidable. By continuing to reward Palestinian terrorism even after September 11, they have continued to make future terrorist attacks more likely.

One explanation, though certainly not a justification, for why some nations deal with terrorism by capitulating, involves a variation on the prisoner's dilemma. This heuristic model involves two hypothetical prisoners who are confined in separate cells awaiting trial. If neither prisoner cooperates with the police, they would both be better off, since there would be no direct evidence to convict them. But if either cooperates, then the cooperator would be better off, because he would get a deal, while the non-cooperator would be convicted and get a harsh sentence. Since neither knows whether the other will cooperate, each must make a decision based on speculation as to what the other will do.

I have had considerable experience with real prisoners' dilemmas, especially in white-collar conspiracy cases involving several suspects. Although each alleged conspirator knows that without the cooperation of one there will be no case, none of them trust the others to stand tall against the authorities who are offering a deal to the first cooperator. Generally, one of them breaks down and the prosecutor gets to make his case against the others. The reason one of them breaks down is that his lawyer tells him that experience shows that at least one will eventually cooperate and that he is better off "winning the race to the prosecutor's door," since the first cooperator gets the best deal and later cooperators may get no deal at all.

How does this prisoner's dilemma relate to terrorism? Every rational nation knows that the international community would be better off as a whole if no nation were ever to give in to terrorists. But every nation also knows that some nations, seeking short-term advantage, will give in. The nations that do give in may be able to strike a deal with at least some terrorist organisations, sparing their citizens from being targeted. In the case of Germany, the Brandt government was even willing to participate in a staged hijacking, in which it knew no one would be hurt, rather than risk a real hijacking that would place its citizens at risk. Other nations have freed imprisoned terrorists, paid cash bounties, or otherwise helped the cause of terrorists. Some have generally refused to submit to terrorists' demands. The reality is that terrorists can count on some countries being more willing to capitulate than others.

These capitulating countries - such as France, Germany and Italy - may in fact be subject to more instances of conditional terrorism, such as hijackings during which no one is hurt, if the government submits to the terrorists' demand to release prisoners. Everyone knows that these capitulating countries will quickly give in to the demands of the terrorists so that it is not to the terrorists' advantage to hurt their citizens. Other countries, which have reputations as non-capitulators, will be subject to fewer instances of pure conditional terrorism but more instances of nonconditional violent terrorist acts, such as blowing up airplanes, shooting passengers and destroying buildings.

Some of these violent acts will be directed against the targets of non-capitulators that are located within the jurisdiction of the capitulators. Examples of this are quite common and include the hijacking of the Achille Lauro containing Americans and Jews, the blowing up of Americans in Germany, and the hijacking of American and Israeli planes from French, Italian or Greek airports. The terrorists know that if they are caught in a country that capitulates, they will be quickly released, even if a noncapitulating country is seeking their extradition. This cynical phenomenon, widely known both to terrorists and to the countries involved, determines - at least in part - the rules of the terrorist game.

Throughout its long history, terrorism has had a checkered record of stunning successes and dismal failures - at least as judged by the achievements of the goals proclaimed by its practitioners. Recently, terrorism's successes have been more visible than its failures, and the international community - diplomatic, religious, academic - has been selective in its condemnation of terrorists. For these reasons, aggrieved groups and individuals have increasingly found terrorism an acceptable, and even attractive, option. Nearly every nation has made some use of terrorism. Nearly every cause has supported or benefited from some form of terrorism. Many individuals have terrorist groups they "approve of" "refuse to condemn" "understand" or apply a more permissive standard to than the one they use for other terrorist groups.

The US has supported, financed and trained groups that are widely regarded as terrorist, such as the Contras in Nicaragua, the mojahedin in Afghanistan, Unita in Angola and Samuel K Doe in Liberia/Sierra Leone. Many Jews supported the Irgun and the Stern Gang during Israel's struggle for independence. The anti-apartheid movement in South Africa employed terrorism, with widespread support from many African-Americans (and others as well). To a far lesser extent, small numbers of blacks supported, or at least refused to condemn, such American terrorist groups as the Black Panthers, the Revolutionary Action Movement and the Black Liberation Army. Some Irish-Americans have assisted Catholic terrorists in Northern Ireland, while some have quietly admired their actions. A small number of Jews supported the Jewish Defence League during the late 60s and early 70s. A substantial number of American Muslims have contributed money to Islamist terrorist groups.

The important point is that terrorism, as a mechanism of change, has never been universally condemned - at least before September 11. Our past policies - which have rewarded the causes of terrorists, honoured their leaders, freed the actual terrorists, neglected on-the-ground intelligence, failed to secure our airports and borders, allowed terrorist groups and states that support them to obtain weapons of mass destruction and refused to hold states that sponsor terrorism accountable for the actions of their surrogates - were prescriptions for creating more and worse terrorism.

No one who observed how the world responded to the globalisation of terrorism between 1968 and 2001 should have been surprised by September 11. The only surprise, and one I suspect the terrorists did not anticipate in light of the world's previous reactions, was the swiftness and massiveness of the military response. We are now, finally, engaged in an effort to send a message to terrorists - at least terrorists who target Americans - that they will pay a heavy price for their terrorism, that terrorism will only hurt their cause.