The First Crusade, called for by Pope Urban II to recapture the Holy Land.
In 1095CE, Pope Urban gathered the leaders of the Christian community at the Council of Clermont. Urged on by Emperor Alexius Comnenos of Constantinople, he called for a crusade to retake the Holy Land from the Muslims who occupied Jerusalem. Muslims had occupied the Holy Land for over 400 years, but the timing was politically right for the Pope and the Byzantine Emperor. Pope Urban wanted to re-unite Christendom after the anti-Pope kicked him out of Rome, while Alexius Comnenus wanted to retake the territory he had recently lost in Anatolia from the Seljuq Turks. As incentive, the Pope offered crusaders a plenary indulgence: complete forgiveness for past sins in the eyes of God and the church. It worked too well. While the official armies of the Crusade prepared, a charismatic leader named Peter the Hermit began breaching directly to the people, claiming Jesus had sent him to lead them on Crusade. Walter sans Avoir joined him in France, and a man named Count Emicho of Leiningen emulated him in Germany. Both peasant groups met with and created disaster: Walter Sans Avoir's group pillaged Belgrade while Count Emicho's group turned on the local Jewish population as an excuse to slaughter them. Thus the First Crusade began with a disastrous People's Crusade.
Emicho of Leiningen and Walters sans Avoir certainly made a mark, but the largest group in the People's Crusade was led by Peter the Hermit. To gain passage through Hungary, they swore an oath not to destroy anything, but the lack of real leadership for their group became clear when they very quickly started a market brawl, stormed the local citadel, then fled to Belgrade and immediately repeated their aggression by turning on the Byzantine troops sent to keep them in line. At the city of Niŝ, the Byzantine troops pinned them down and slaughtered a quarter of the entire crusading "army." The remainder fled to Constantinople and secured passage into Turkey, but the group fractured from within and became two separate factions, with Peter leading one and a man named Reinald leading the other. Both factions competed for bragging rights, committing horrible atrocities to outdo each other. One group actually managed to siege a castle, but it had no water supply, so they were easily starved out by the Turks. The Turks, however, spread a rumor that this group had actually gone on to capture the capitol city, and the remaining crusaders set out to join what they thought would be a loot extravaganza. Instead, they ran into a Turkish ambush that left only 3,000 of their 20,000 soldiers alive. Now led by Geoffrey Burel, they retreated to Constantinople.
Although it finds Peter the Hermit's group from the People's Crusade in shambles, the summer of 1096 finally sees the "official" forces of the First Crusade set out for Jerusalem. This was not one army, however, but five separate armies led by men with very different motivations and sympathies - many of them surprisingly hostile towards the Pope or the Byzantine Empire. Hugh of Vermandois, brother of the King of France, led one army despite his brother having been excommunicated by Pope Urban II. Godfrey de Bouillon from the German territory had actually helped kick the Pope out of Rome and install the anti-Pope. Bohemond of Taranto brought an army whose experience primarily came from fighting the Romans twelve years prior. Raymond of Toulouse led the largest army and believed himself the main leader of the Crusade, despite the fact that he traveled with the Pope's appointed leader, Bishop Adhemar. Only Robert of Flanders could be said to be on good terms with both the Pope and the Eastern Roman Empire. When the five armies arrived in Constantinople, Emperor Alexius Comnenus approached them all privately with bribes and threats to get them to swear an oath that any land they conquered on Crusade would be returned to him. They all took it (except Bohemond's nephew, Tancred) and so the emperor sent them across the Bosphorus to attack the Turks at last.
Having sworn their oaths to Emperor Alexius Comnenus, the Crusaders finally sailed across the Bosphorus River to Turkey. When they disembarked, however, there were no Turkish armies waiting for them. Unopposed, they marched to Nicaea, the capitol of the Sultanate of Rum, and laid siege to it. At last word reached the sultan, Kilij Arslan, who rode back to save his city (and his family) only to realize that this army of crusaders was much larger and better organized than the People's Crusade which had come before. They had not yet realized, however, that the city of Nicaea was being secretly resupplied by ships arriving by night from Lake Askania. Once they did, the Byzantines transported their own ships overland to blockade the lake and launch a coordinated assault with the crusaders to force the city to surrender. The crusaders marched towards Jerusalem, but along the way, the Turks launched a surprise assault on Bohemond's army. He ordered his knights to form a shield wall around the priests and civilians traveling with them, and they held for hours under a burning sun until reinforcements from the other crusading armies arrived. They rallied, defeated the Turks, and resumed their march.
After their victory at the Battle of Dorylaeum, the Crusaders have an open path to Antioch and beyond that, Jerusalem. After the Sultan of Rum, Kilij Arslan, ordered the wells destroyed along their path, the Crusaders struggled through the desert and eventually decided to split their forces. Tancred and Baldwin set off towards Tarsus and Tancred tricked the Turkish garrison into surrendering to him, but Baldwin claimed the city for himself and broke his oath to the Byzantine Emperor Alexius Comnenos. Tensions between the two lead to another confrontation in the next city, after which Baldwin abandoned the Crusade entirely and conned his way into becoming the Count of Edessa. Tancred meanwhile returned to the main force of Crusaders, who were besieging Antioch. When a force led by Bohemond and Robert of Flanders met Antioch's Turkish reinforcements on a foraging mission, they attacked them and scared them away. Then Bohemond tricked the Byzantine general into leaving as well, and threatened to leave himself unless the Crusaders let him keep Antioch. They had no choice but to agree to keep their forces together. With this assurance, Bohemond engineered the capture of Antioch: he bribed a Turkish commander to let them through the gates. The Crusaders massacred the people of Antioch when the city fell, but they had no time to rest after their victory: a huge Turkish army was already bearing down on them.
The Crusaders now held Antioch, but not securely. The Turks still control the citadel atop the mountain and had a massive army coming to reinforce them. The situation grew worse when Stephen of Blois deserted from the Crusades, and told the Byzantine reinforcements not to bother: he believed Antioch would fall immediately. Now entirely on their own, the Crusaders held the wall in constant vigil until a mystic named Peter Bartholomew claimed to have received a vision from Saint Andrew. Guided by his vision, he discovered metal which he claimed to be the holy lance of Longinus - nevermind that the church already had the holy lance in its possession. Though the Crusade leaders had doubts, the soldiers were inspired so they launched an assault on the Turkish armies. Surprisingly, they won the day: the Turks did not fully support their leader, Kerbogha, and many took the Crusade counter-attack as an excuse to abandon the siege. Bohemond now kept Antioch, while Raymond of Toulouse - after the disastrous Siege of Maarat led the soldiers to commit acts of cannabalism - took the remains of the army south to Jerusalem. His attempt to capture a small city called Arqa along the way almost fractured the crusade army again, and did lead to the death of Peter Bartholomew. They arrived in Jerusalem to find the local wells poisoned, giving them no choice but to attack the city head-on. After days of intense fighting, they won their way inside the walls and began a massive slaughter of the people who still lived inside Jerusalem - the Christian population had been expelled, leaving only Muslims and Jews still in the city. And thus, with Antioch and Jerusalem both in crusader hands, the First Crusade came to an end.