The First Crusade was a military campaign initiated by Western Christendom between 1096 and 1099 to recover the Holy Land of Levant, occupied during the Muslim conquest, and culminated in the 1099 Crusades' capture of Jerusalem. According to Christian legend, Palestine was the place where Jesus was born and ascended to heaven, and his tomb is in Jerusalem, which is considered a holy place. Jerusalem was the political and religious center of the Jews in ancient times, the capital of the Hebrew kingdom, and naturally considered a holy place by Jews. According to Islam, the Allah emissary Muhammad ascended to heaven on a horse in Jerusalem on July 17, 622, and that day was designated by the Islamic calendar as the Feast of The Ascension, and Jerusalem became a holy city for Muslims.

The human-faced Flying Horse Brag on which Muhammad ascended to the throne
In the mid-11th century, the rise of the Islamic Seljuk Turks and their western conquests complicated the situation in the Near East. They controlled almost all of Asia Minor, but instead of forming a unified state, they were divided into separate governorates such as Roma, Mosul, Damascus, Aleppo, Antioch, Tripoli, etc.
In the 1690s, infighting broke out among the Seljuk Turks, and some Islamic mosques were destroyed, but pilgrims from the sea could still go to Jerusalem after paying a small amount of tax. However, the Pope and the Holy See fabricated news that Muslims insulted Western pilgrims and incited religious antagonism. At the same time, the vast Byzantine Empire was in decline. In 1091, a turkic force prepared to attack the Byzantine capital of Constantinople, and the empire was in danger. The desperate Emperor Alexei I (1081-1118) had to appeal to the Pope and the Holy Roman Emperor for help. The plight of the Byzantine Empire finally became an excuse for the feudal lords of Western Europe to wage wars of aggression.
In 1095, Pope Urban II (reigned Pope 1088-1099) called for the formation of a crusade at the Congregation of Clement in France. At the end of the meeting, he delivered an impassioned and inflammatory speech to the people. He chronicled the suffering of Christians in the East and the "atrocities" of the Turks, calling on greedy lords, belligerent knights, adventurous merchants, and blind peasants to take up arms and go to the East to reclaim the Lord's grave from the pagans. He promised that anyone who took part in the expedition would be forgiven of their sins and ascended directly to heaven after death. The crowd was in a state of excitement, and Urban's speeches were from time to time called "Amen! "Amen!" (Hebrew, meaning only wish) was interrupted by a cry. The impulse of religious feelings, the temptation of material interests, led many strata of people in Western Europe, despite their different aims, to throw themselves into the war of conquest.
The four main crusaders chose different routes of march, and originally the crusade was scheduled to depart on the day of the Ascension of the Virgin Mary, that is, On August 15, 1096, but in February of that year, a detachment of twenty thousand poor peasants was already eager to embark on the journey, and this crusade was composed of 20,000 poor peasants, called the "Peasant Crusade". They traveled through Hungary and Byzantium to Asia Minor. However, soon after the crusade, which was mainly peasants, thousands of people died of illness on the way. This army was undisciplined and committed atrocities of forced conversion to Christianity and the killing of Jews. Although the army reached Constantinople, it was slaughtered by the Seljuks in Asia Minor. Another crusade, organized by the European nobility, set out on a crusade in August of the same year. In April 1097, the crusaders reached Constantinople, received supplies from the Byzantine Emperor Alexei I, and crossed the sea into Asia Minor. In June 1097, the Crusaders captured Nicea, and on July 1 of the same year, the Crusaders inflicted heavy losses on the Seljuk Turks at Dorylaeum. The Crusaders then took advantage of the victory to march into Syria and capture Edessa. In June 1098, the Crusaders besieged and captured the city of Antioch. From January to June 1099, the Crusaders marched on Jerusalem.
On June 7, 1099, the Crusaders set up camp under the city of Jerusalem. Egyptian officers in the city reinforced the city's defenses and ordered the expulsion of the Christian inhabitants, holding the walls and clearing the wilderness, and waiting for help. In fact, the Crusaders had only 1,500 knights and 12,000 infantry, and the egyptian mainland had much larger reinforcements.
Without siege machinery, the Crusaders were unable to attack effectively. The general offensive on 12 June failed and suffered heavy losses. They had to build siege machines. Because of Jerusalem's desert location, they spent weeks traveling to Samaria to cut wood.
In July, news came that the Egyptian native army had finally set out, and now the Crusaders had only 1 month to capture the city. It was time to exert religious power, and the priests, claiming to have received a divine instruction from the dead Bishop Ahimal, ordered the whole army to fast, and after fasting, on July 8, the whole army, dressed in the white cloth of the repentant, sang hymns, and began to walk around the city walls with solemn steps. The Muslims in the city had never seen such a sight and laughed at it. After the procession, the Crusaders ascended mount of Olives to listen to the sermon of the hermit Peter.
Over the next 2 days, the Crusaders built 3 siege towers, advanced their positions, and began bombarding the city walls. On the night of the 13th, the siege began again. After a day and a half of fierce fighting, in the middle of the night on the 15th, in the north, they built a bridge from the tower to the city wall. After they occupied a section of the city wall, the people under the city set up ladders. They opened the GateoftheColumn and went straight to the DomeoftheRock to accept the surrender of the Egyptian officers and hung the Crusader flag on top of the mosque. To the south, however, the Crusaders were still fighting fiercely, and the fierce fighting continued until the middle of the night.
After that, the massacre began, and the commanders lost control of the army. After days of fasting, religious rendering, and bloody battles, the Crusaders had gone mad. They kill the infidels when they see them, and they rob their belongings when they see them. Large numbers of Muslims gathered in the mosque of the Dome of the Great Mosque, al-Aqsa. On the morning of the 16th, a group of crusaders broke through the door and killed the refugees. The Jews hid in the synagogue and were set on fire.
On the 17th, order was finally restored to the city as the generals took control of the troops and the Muslim and Jewish residents were killed. The Christian inhabitants had previously been expelled, and now all that remained was an empty city for the conquerors.
The First Crusade established four "Crusader States", namely the Kingdom of Edessa, the Principality of Antioch, the Kingdom of Jerusalem and the Tripolit State in Palestine, and the Kingdom of Armenia, composed of Armenian refugees, became allies of the Crusaders. The First Crusade was a Christian response to the expansion of Muslim power, and in the nearly two hundred years that followed, the Second Crusade to the Ninth Crusade came. At the same time, the Crusades indirectly reopened international trade that had declined since the fall of the Western Roman Empire.