Assassination may be the most extreme form of censorship, but it is not necessaily the most effective. Political murder changes history, but it seldom changes minds.

America would not be the same place today if John F. Kennedy had lived. The murders of Anwar Sadat and Yitzhak Rabin are central to any understanding of the course of modern Middle Eastern history. The world would be quite different if Reagan had been shot and killed, or Lincoln had not.

Yet it is undoubtedly true that political assassination rarely achieves the goal the assassin hopes for, and sometimes produces effects that are the reverse of those intended. Quite often, an assassination provokes outcomes that are entirely unpredictable: Gavrilo Princip certainly wanted to kill Archduke Franz Ferdinand in 1914; he did not intend to start a world war.

Whether the repulsive murder of Benazir Bhutto has the effect her killers intended depends on many factors that are now entirely beyond their control. Assassins seek to revise future history, to shape it in specific directions; they act from motives that are partly political, sometimes personal, and not infrequently deranged. Some assassins are seeking immortality; more often they confer immortality, in the form of martyrdom, on their victims.

Most assassins are swiftly forgotten. Who now remembers the names of the killers of Indira or Rajiv Gandhi? As an individual John Wilkes Booth, the assassin of Abraham Lincoln, is little more than a footnote. The name of the Hindu extremist who killed Mahatma Gandhi is not even mentioned in most Indian biographies. Lee Harvey Oswald won celebrity, but only because he, too, achieved a sort of perverse martyrdom among conspiracy theorists by being killed.

The word assassin has its origins in the Nizaris, a violent Islamic sect that emerged at the end of the 11th century in what is now northern Iran; they served as contract killers for political masters, usually after taking large amounts of drugs. Assassin derives from hashashin, Arabic for hashish.

Many assassins are simply anachronisms, seeking to reverse history when events have already moved on. Wilkes Booth hoped that killing Lincoln would reopen the wounds of the American Civil War (it didn't, and may have helped to salve them); Yigal Amir, who killed Rabin in 1995, was attempting to derail the peace process (he didn't, and Rabin's successor, Shimon Peres, accelerated it); Sirhan Sirhan, a Palestinian Arab, killed Bobby Kennedy in apparent retaliation for his support for Israel (but the Democratic Party's policy towards Israel was not merely unchanged, but reinforced); John Hinckley tried to kill Ronald Reagan because he thought it might make the actress Jodie Foster fall in love with him (needless to say, she did not).

The killers of Benazir Bhutto may find that instead of destroying the movement she headed, they have draped her in a martyr's mantle. Lincoln's murder ensured that, for successive generations, he was above historical criticism. The killing of Martin Luther King lent the civil rights movement an unstoppable moral momentum.

It takes a subtle mind, far beyond the reach of the fanatic, to realise that political murder seldom works. When the opportunity to assassinate Hitler presented itself in 1944, Churchill sensibly demurred. In part his reasons were personal, and partly moral, since the killing of Reinhard Heydrich, the Nazi ruler of Czechoslovakia, had unleashed reprisals on a terrifying scale. But mostly, his thinking was pragmatic: Hitler was already losing his grip, and Himmler, potentially an even worse enemy, was waiting in the wings to take over.

Terrorists, by definition, are concerned only with brute revenge and instant fear, with destroying a hated enemy, not the subtleties of cause and effect. If, say, the IRA had managed to kill Margaret Thatcher in the Brighton bombing of 1984, the Tory Government would never have contemplated talks involving the IRA by 1995; Republicans would have been shut out from negotiation indefinitely, and perhaps for ever.

But terrorists do not think so far ahead; like the original Nizaris, high on their own self-righteousness, the killing is all that matters.

Bhutto's mourning supporters may reflect that the brutes of history are seldom assassinated. Stalin, Mao, Hitler, Pol Pot and Saddam had the tyrannical apparatus to ensure that the assassins did not get close. It is the brave and prominent public individuals, in an open society, who are most vulnerable to assassination: the Kennedys, the Gandhis, Lincoln, King and now Benazir Bhutto.

Whether yesterday's killing derails Bhutto's cause depends on the reaction of the Pakistani authorities, how swiftly and reliably her killers can be identified and, perhaps above all, who now steps up to take her place. The subcontinent has a horrific history of assassination, but also of recovering with extraordinary resilience: Sonia Gandhi followed Rajiv Gandhi followed Indira Gandhi. Solomon Bandaranaike, the murdered Prime Minister of what was then Ceylon, was soon succeeded by his wife; Benazir Bhutto followed her father, executed in 1979, and someone will follow her.

“Anyone can kill anyone,” said Lynnette “Squeaky” Fromme, the acolyte of Charles Manson, who came close to assassinating Gerald Ford. In troubled Pakistan, assassination still seems horribly easy. The course of history has been changed by it, once again. But killing someone is easier than killing what they stood for, as those who plotted Bhutto's murder yesterday, and applaud it today, may soon discover.

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