Nobody at Clermont in 1095 used the word crusade. It does not appear in any language for roughly a century after the events, and the people involved described what they were doing as a pilgrimage, or simply as taking the cross.
That matters, because pilgrimage is the frame they were working in. Armed pilgrimage was a contradiction the church had spent centuries developing the theology to resolve, and the resolution was that killing in a war authorised by God was not merely permitted but meritorious, and that going on the expedition could substitute for the penance owed for a lifetime of sins.
For a knight this was a remarkable offer. His profession was violence, the church had spent two hundred years telling him that violence imperilled his soul, and he was now being told that the violence itself, directed correctly, was the remedy. The response rate suggests the anxiety was real.
It was also enormously expensive. A knight had to fund himself for a journey of unknown length, and many mortgaged or sold land to churches and monasteries to do it. The charters recording those sales survive in quantity and are among the best evidence we have for who went and what they expected.
The immediate occasion was a request for help. The eastern emperor, having lost the Anatolian interior to Turkish groups after a catastrophic defeat and a worse civil war, asked the pope for western mercenaries.
What he got was not what he asked for. Urban the Second preached at Clermont in November 1095, and five versions of the sermon survive, all written after the expedition had already succeeded, all differing, none reliable. What they agree on is an appeal combining the sufferings of eastern Christians, the defilement of the holy places, the promise of remission of sins, and an invitation to men who were fighting each other to go and fight somebody else.
The papal motive is not mysterious in the light of the previous chapter. A pope who could summon the armed nobility of Europe and direct it was a pope whose claims about his own authority were being demonstrated rather than argued.
The response exceeded anything intended. Before the organised armies moved, popular preachers gathered large bodies of ordinary people who set off early, unsupplied and undisciplined. On the way through the Rhineland they attacked the Jewish communities of Speyer, Worms, Mainz and Cologne, killing thousands. Local bishops tried, in several cases, to protect them and failed. This was not incidental. It was the first mass killing of Jews in western European history, it established a template, and it happened before a single crusader reached the east.
The main armies, perhaps sixty thousand people in total including non combatants, travelled overland through the Balkans, extracted an oath from the eastern emperor that they would return conquered territory to him, and crossed into Asia Minor in 1097.
What followed was three years of extreme hardship: a march across Anatolia in summer, an eight month siege of Antioch in which the besiegers starved, then a reversal in which the newly captured city was itself besieged, then a further year of delay while the leaders quarrelled over who would keep what.
They took Jerusalem in July 1099, and killed a large part of the population, Muslim and Jewish, over two days. Christian and Muslim sources agree on a massacre and disagree on the scale, with the crusaders' own accounts if anything boasting of it in language drawn from the Book of Revelation. Modern estimates put the dead in the thousands rather than the tens of thousands the medieval sources claim, which is not a defence of anything.
The reason it succeeded was largely luck of timing. The Islamic world at that moment was fragmented between rival Turkish emirs and a hostile Egyptian caliphate, with no coordinated response available. Nobody in the region seems initially to have understood the expedition as a religious war; the earliest Arabic accounts treat it as another Byzantine incursion.
The crusaders then set up four small states along the coast, held them with a permanently insufficient number of western settlers, and defended them with castles and with two new religious orders of warrior monks who took monastic vows and fought as disciplined heavy cavalry.
Everything after the first expedition is a record of diminishing returns.
A second, launched after one of the crusader states fell, was led by a king of France and a German emperor, achieved nothing, and ended in a botched attack on Damascus, which had been a friendly power.
A third followed the recapture of Jerusalem in 1187 by Saladin, who had united Egypt and Syria and destroyed the crusader field army at the Horns of Hattin. It brought the three greatest rulers of Europe east. One drowned in a river in Anatolia. Two quarrelled. The English king won his battles, failed to take Jerusalem, and negotiated a treaty guaranteeing pilgrims access, which was arguably the most rational outcome anybody achieved in two centuries.
The fourth is the one that should be remembered. Assembled in Venice, unable to pay the Venetians for the fleet they had ordered, the army agreed to work off the debt by capturing a Christian city on the Adriatic for Venice, and was excommunicated for it. Then a claimant to the Byzantine throne offered them money and troops if they would install him. They did. He could not pay. In April 1204 they stormed Constantinople, sacked it for three days, destroyed or melted a large part of the accumulated art of a thousand years, and set up a Latin emperor. The pope who had launched the expedition denounced it and then accepted the result.
After that: a crusade against heretics in southern France that killed on a large scale and destroyed a regional culture, expeditions to Egypt on the sound reasoning that Egypt was where the power was, and which failed on logistics and on the Nile flood, and a long slow reduction of the coastal states until the last of them fell in 1291.
Between the campaigns there were two centuries of ordinary life in the crusader states, and it looked very little like the sermons.
The Frankish population was always tiny relative to the people it governed, who were mostly Muslim peasants and eastern Christians of several denominations, plus Jews and Samaritans. A settler elite that thin cannot rule by extermination even if it wants to. So villages kept their own headmen, their own courts for their own affairs, and their own religion, and paid the new landlords the dues they had paid the old ones. Muslim law books and Christian accounts agree that mosques went on functioning.
The frontier was permeable in every direction. Frankish lords made treaties and tribute arrangements with Muslim neighbours and broke them on the same calculations they would have used at home. Italian merchants traded straight through the wars. Diplomatic marriages, hostage exchanges and joint campaigns against third parties were all normal.
The best source for the texture of it is a memoir by a Syrian gentleman named Usama ibn Munqidh, written in his eighties, who fought Franks, negotiated with Franks, hunted with Franks and found them fascinating and ridiculous. He records their medicine with horror, their jealousy or lack of it with astonishment, and notes that the ones who had settled and grown up in the country were much better company than the ones freshly arrived from Europe, who were dangerous because they did not know the rules.
That distinction, between the settlers and the new arrivals, appears on the Christian side too and it is the political history of these states in miniature. The people who lived there wanted arrangements. The people who came for a season wanted a holy war. The arrivals were usually more powerful and always more numerous when they came, and they wrecked several workable settlements.
Take the claims in turn, because the popular ones are mostly wrong.
It did not bring Greek learning to Europe. That came through Spain and Sicily by translation, as chapter ten described, and through Constantinople by trade and by scholars fleeing later. The crusaders were not a scholarly population.
It did not open trade with the east. That trade already existed and was run by Italian merchants who continued it throughout, including with the enemy, in defiance of repeated papal bans. What the crusades did was strengthen Venice and Genoa enormously, because armies needed shipping and shipping was where the money was. Venice's position after 1204 gave it a commercial empire that lasted three centuries.
It did not, on the whole, win land for younger sons. Most who survived came home, and many came home poorer than they left.
It did not permanently damage Islamic power. It provoked a consolidation, and the states that emerged, Saladin's and then the Mamluks of Egypt, were stronger than the ones the crusaders had originally faced.
What it did do is harder and worse. It normalised religious war within Christendom, since the same machinery was turned on heretics, on pagans in the Baltic, on political enemies of the papacy, and on Muslims in Spain. It made violence against European Jews a recurring feature rather than an aberration. It supplied both Christian and Muslim traditions with a memory of the other as an invader, and that memory has been reactivated deliberately by politicians in both directions in the last two centuries, usually with a version of the history that no specialist would recognise.
Firm: the chronology, the routes, the sieges, the states founded, and the charters. The property transactions that funded departures are excellent evidence and are not shaped by anyone's narrative.
Soft: motives. The older view that crusaders were younger sons seeking land has been largely overturned by the charter evidence, which shows men selling land to go and frequently returning poorer, if at all. The religious motive appears to have been genuine and primary for most participants, which is more disturbing than the cynical reading rather than less.
Soft: numbers throughout. Army sizes and casualties in medieval sources are conventional figures, not counts.
Soft: the sermon at Clermont. Every version we have was written afterwards by someone who knew how it turned out.
Contested: whether the whole category holds together. Some historians restrict the term to expeditions to the Holy Land; others include every papally authorised war with the same privileges, which extends the phenomenon into the fifteenth century and across Europe. How you define it determines what you conclude about it, which is worth knowing when you meet a confident summary.
One correction to a widespread modern claim: the Islamic world did not spend the following centuries brooding on the crusades. Arabic writing on them is relatively sparse until the nineteenth century, when the memory was revived, in part in response to European colonialism and in part by reading European histories. The grievance is real; its continuity is not.