From papal-backed campaigns in the Middle East to temperance leagues and the War on Terror, the Western idea of the 'crusade' has mutated but endured. Jonathan Phillips on a history of holy and profane struggles.

In the emotional aftermath of the terrorist attacks of 9/11, President George W. Bush began to articulate his response: "This ... (long pause) crusade ... this War on Terror is going to take a while."

Thus he offered up one of the most controversial remarks of recent years. His choice of the word "crusade" was a propaganda gift to Osama bin Laden, who could argue that, just as crusader forces in medieval times unleashed death and destruction on the Muslim world, now Bush had called for the process to be repeated. While the validity of this analogy is often disputed, it is worth considering how and why the idea of a "crusade" has survived in the West to this day. With the principal exception of the First Crusade's capture of Jerusalem in 1099, after all, the majority of medieval crusading activity was unsuccessful.

For the concept, or some mutated descendant of it, to remain of such significance demonstrates its ability to compress an extraordinarily potent set of messages. Yet such is its flexibility that it can move from a context that is highly incendiary to one that is utterly mundane.

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Away from the charged arena of modern politics, the word is frequently used with barely a glance at its origins. Anthea Turner described herself in a recent BBC3 series as being "on a crusade to help people on the brink" of rodent infestation, while the actor Jon Pertwee claimed that Dr Who was "an intergalactic crusader". So what is now understood by the word "crusade", and does it bear any resemblance to its original meaning?

A medieval crusade could be defined as a holy war initiated by the Pope on God's behalf in which the participants took the cross and received remission of all their sins. In theory, therefore, it was a penitential exercise. The First Crusade ushered in a period of almost 200 years of Christian rule in the Levant. Yet in time crusades were directed against a variety of opponents, not just Muslims, and went to places other than the Middle East. Crusaders fought Cathar heretics, political enemies of the papacy, the Mongols and the pagan tribes of northern Europe. The crusading ideal persisted until the 15th and 16th centuries, with popes regularly calling for campaigns against the Ottoman Turks.

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By the time of the Enlightenment, however, the notion of fighting for one's faith was falling into disrepute. Crusading was decried as "a singular monument of human folly" by the Scottish historian William Robertson, while Edward Gibbon's History of the Decline and Fall of the Roman Empire (1776-88) argued that "the principle of the crusades ... had checked rather than forwarded the maturity of Europe".

Such attractive soundbites have been repeated frequently by historians and they give the impression that this primitive, medieval idea, born out of superstition and barbarity, was worthless and dead.

Yet the cultural engines of Romanticism and Orientalism gave the memory of the crusades a significant boost in the 19th century. Of course, the Romantic version of history largely ignored the brutality of the crusades and emphasised exoticism and, perhaps more important, nobility and moral right. The immensely popular works of Sir Walter Scott (including Ivanhoe in 1819 and The Talisman in 1825) caught the imagination of generations of readers. The arts produced crusade-themed works: Edvard Greig composed Sigurd Jorsalfar, or Sigurd the Crusader; while operas by Meyerbeer, Verdi, Rossini, Schubert and Spohr were performed across Europe.

The rise of so-called muscular Christianity in the Victorian period also encouraged crusading themes, as evidenced by hymns such as Onward, Christian Soldiers (1864). An obvious visual reminder is the splendid equestrian statue of Richard the Lionheart commissioned by Prince Albert to stand outside the Houses of Parliament.

In conjunction with a romanticised view of crusading, the notion of moral right, buttressed by the sanction of proper (and often royal) authority, helped to drive the expansion of European power through imperialism and colonialism.

In France, for example, by the early decades of the 19th century the history of the crusades seemed to offer a sufficiently close parallel for its rulers to openly invoke the medieval age as a forerunner of their own enterprises. During the French invasion of Algeria in 1830, the expedition's commander, General Bourmont, recalled the great crusade of Saint Louis in 1248-54 and suggested that he was setting out on his own crusade.

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Underpinning such thoughts was Joseph Michaud's hugely influential Histoire des croisades (19 editions were published between 1808 and 1899) that was also read, in a special edition, by generations of French children and, broadly speaking, provided a ringing endorsement of crusading as a source of glory and achievement for the nation.

When it came to the First World War, the emotive pull of the crusade formed one element of the propaganda effort against the Germans. Anglican clergy such as the Bishop of London spoke of "a great crusade ... to save the world". Lloyd George made a speech at Conway in 1916 in which he claimed men were flocking to join "a great crusade". General Allenby's capture of Jerusalem in 1917 was the most obvious episode to be portrayed in this way, with a famous Punch cartoon depicting Richard the Lionheart gazing at Jerusalem with the caption "At last my dreams come true" - a reference to the king's failure to take the city during the Third Crusade (1189-92). Allenby himself disliked the crusading comparison, not least because a large number of his troops were Egyptian Muslims and he did not wish the struggle to be viewed as one of Christianity against Islam.

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Probably the most intensive use of crusading themes in recent centuries was in Spain. Crusading in Iberia had, unlike many ventures elsewhere, succeeded through the final expulsion of the Muslims in 1492. By the 1930s General Franco, in conjunction with the Catholic Church, often used crusading imagery in his struggle against the Republicans and the Left. When, in 1939, the Pope congratulated Franco on his victory, he wrote that "a heroic crusade has been fought against the enemies of the fatherland and Christian religion".

It was not just in military situations that the idea of crusading persisted. One of the founders of the Women's Temperance Crusade in 1870s America, Mildred Carpenter, wrote of "a fight against organised evil" and argued passionately that "it is a glorious heritage to leave our children, to be able to say 'I was a crusader in Washington Court House'". She described one of the movement's preachers as an "Apostle of Temperance", his followers as "aflame with the Master's zeal", and the whole episode as "a whirlwind of the Lord".

Similarly, the Jarrow Crusade for jobs on Tyneside in 1936 has some interesting parallels with medieval crusading, not least in the sense of a pilgrimage with a goal - in this case the very secular destination of the Houses of Parliament, rather than the Holy Sepulchre. There was, however, a religious aspect to the march when the crusaders received the blessing of the Bishop of Jarrow and many Church of England clergy also offered their support.

The Second World War saw further use of crusading imagery, but since 1945 and the decline of nationalism and imperialism the idea of crusading has seemed less applicable to military situations. The vast scale of the slaughter in 20th-century warfare and the utterly impersonal nature of the destruction wrought by nuclear bombs have rendered parallels with brave adventurers somewhat redundant.

Furthermore, as the Muslim world - both Islamists and more secular anti- Western regimes such as Libya in the 1980s - awoke to the parallels of history, they have highlighted the harsher realities of crusading in a way that has proven highly effective in mobilising support.

All this is not to say that the word "crusade" has fallen out of use in a secular environment, as we have seen, but politicians have learnt to be wary of it. In his last months as Prime Minister, Tony Blair was asked on the Today programme if he saw himself as "a crusader" for social reform. Given the controversy of the Iraq war and the issue of his own spirituality, he carefully avoided taking the bait and stated simply that he was concerned to improve social justice. Accepting the label "crusader" was just too easy an invitation to a PR disaster.

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Jonathan Phillips is professor of crusading history at Royal Holloway, University of London. He is the author of The Fourth Crusade (2004) and The Second Crusade: Extending the Frontiers of Christendom, recently published by Yale University Press.