The Crusades: Map of the First Crusade and the Crusader States

In the 11th century the Seljuk Turks swept across the Middle East. They crushed the Byzantine army at Manzikert in 1071 and overran most of Anatolia, and in 1095 the Byzantine emperor Alexios I Komnenos asked the pope for Western soldiers to help win it back. Pope Urban II turned the request into something far bigger. On 27 November 1095, at the Council of Clermont in France, he called on the knights of Europe to march east, free the Christians of the East and recover Jerusalem, promising forgiveness of sins to those who went.

The response astonished everyone. Tens of thousands of people took the cross, sewing a cloth cross onto their clothes as a sign of their vow. Their motives mixed sincere piety, the hope of salvation, family loyalty, adventure and, for some, land. Jerusalem itself was then in the hands of the Fatimid caliphs of Egypt, who had taken it from the Seljuks in 1098 while the crusaders were still on the march.

The first to leave were crowds of poorly armed pilgrims led by preachers such as Peter the Hermit, the People's Crusade. Bands of them murdered Jews in the Rhineland towns of Speyer, Worms and Mainz in May 1096, and the Turks destroyed most of the rest at Civetot in Anatolia that October. The main armies, led by great nobles such as Godfrey of Bouillon, Raymond of Toulouse and Bohemond of Taranto, set out in the late summer of 1096 by separate routes through Hungary and the Balkans or across the Adriatic, and gathered at Constantinople over the winter. The red lines on the map follow them.

In 1097 the army, perhaps 60,000 to 100,000 people with up to 10,000 knights, took Nicaea, beat the Seljuks at Dorylaeum and crossed the burning plateau of Anatolia. Baldwin of Boulogne broke away to take over Edessa in March 1098, founding the first Crusader state. The main force besieged Antioch for eight months, took it on 3 June 1098 and then, starving, beat a huge relief army under Kerbogha of Mosul. In June 1099 the survivors reached Jerusalem. They stormed it on 15 July 1099 and massacred much of the population, Muslims and Jews alike, a slaughter remembered on both sides for centuries.

The crusaders divided their conquests into four states, which the map draws from 1099: the County of Edessa in the north, beyond the Euphrates; the Principality of Antioch; the County of Tripoli, completed in 1109; and the Kingdom of Jerusalem, whose first king, Baldwin I, was crowned in 1100. Westerners called them Outremer, "the land overseas". They were a thin strip of territory, rarely more than 100 km wide, held by a small Frankish nobility ruling a mostly Muslim and Eastern Christian population, and defended by castles such as Krak des Chevaliers and by the new military orders, the Knights Templar and the Knights Hospitaller.

Italian merchant cities, Venice, Genoa and Pisa, supplied the fleets that took the coastal towns and won trading quarters in return. Compare 1099 with 1140 on the map to see the states at their largest.

The Muslim counter-attack began in the north. Zengi, ruler of Mosul and Aleppo, captured Edessa in 1144, and the Second Crusade (1147–1149), led by King Louis VII of France and King Conrad III of Germany, collapsed after a failed siege of Damascus in 1148. Zengi's son Nur al-Din and then the Kurdish general Saladin united Syria and Egypt. On 4 July 1187 Saladin destroyed the army of the Kingdom of Jerusalem at the Horns of Hattin, near the Sea of Galilee, and on 2 October 1187 Jerusalem surrendered to him. Use the 1188 button to see how little of the kingdom was left.

The Third Crusade (1189–1192) brought the three greatest kings of Europe east. Emperor Frederick Barbarossa drowned crossing a river in Anatolia in 1190; Philip II of France and Richard I of England retook Acre in July 1191 and Richard beat Saladin at Arsuf, but never tried to besiege Jerusalem. The Treaty of Jaffa in September 1192 left the crusaders a coastal kingdom from Tyre to Jaffa, with Acre as its capital, and let Christian pilgrims visit Jerusalem.

The Fourth Crusade never reached the Holy Land. Unable to pay Venice for its fleet, the crusaders first attacked the Christian city of Zara on the Adriatic for the Venetians in 1202, then sailed to Constantinople to put a Byzantine claimant on the throne. When he failed to pay them, they stormed and sacked the city in April 1204, plundering its churches and carrying off relics and treasures, among them the bronze horses now in St Mark's, Venice. They set up a Latin Empire that lasted until 1261; move the map to 1206 to see it. The sack deepened the split between the Catholic and Orthodox churches and left Byzantium too weak to stop the Ottomans later.

Later crusades aimed at Egypt, the heart of Ayyubid power. The Fifth Crusade took Damietta in 1219 but was trapped by the Nile flood in 1221. Emperor Frederick II won Jerusalem back by treaty with Sultan al-Kamil in 1229 without a battle, but it was lost for good in 1244. King Louis IX of France was defeated and captured at Mansurah in 1250, and died of disease at Tunis on a last crusade in 1270.

By then the Mamluks, slave soldiers who had seized power in Egypt in 1250 and stopped the Mongols at Ain Jalut in 1260, were taking the crusaders' castles one by one. Sultan Baybars took Antioch in 1268, Qalawun took Tripoli in 1289, and on 18 May 1291 his son al-Ashraf Khalil stormed Acre after a six-week siege. The last mainland towns were abandoned within weeks, ending almost two centuries of crusader rule in the Holy Land.

The crusades did not keep Jerusalem, but they changed both worlds. They widened trade between Europe and the Levant and enriched the Italian maritime republics, spread the idea of holy war against other enemies, from the Moors in Spain and pagans in the Baltic to heretics in southern France, and left deep scars in relations between Latin Christians, Orthodox Christians, Muslims and Jews. Crusading itself continued for centuries, against the Ottomans as late as the 16th century.