Alexios I Komnenos had not attained the throne of the Byzantine Empire through the legitimate method of inheritance. He had seized the throne in a military coup in 1081. He was appointed as a general by Emperor Michael VII Doukas 1071. Since Alexios had served with distinction in campaigns against the Seljuk Turks, Nikephoros III Botaneiates, the next Emperor who took power in 1078, retained him as a general.
In 1081, Alexios was entrusted with a significant military to counter the Norman threat, but he used this military to besiege Constantinople. On 1 April 1081, Alexios and his men broke through the walls of Constantinople and sacked the city. Botaneiates was forced to abdicate and retire into a monastery where he spent the rest of his life as a monk.
Alexios became the emperor but a significant part of the Byzantine political establishment despised him. They viewed him as a traitor who had acquired the throne through a coup. During his reign, Alexios was battling external threats from the Seljuk Turks and other Islamic forces and internal threats from his political enemies in Constantinople. He maintained his grip on power by promoting his family members to key positions in the government. But the questions regarding the legitimacy of his government would not go away.
By the 1090s, his position in Constantinople had become precarious. He could not be sure of the loyalties of even his family members. Since he could trust no one in his kingdom, he had to look westwards to save his throne. In 1095, he sent his envoys to Pope Urban II to plead for military assistance.
If Alexios needed military assistance then why did he choose to plead before the Pope, the leader of a religious institution, which does not maintain a military. Alexios had good connections with the monarchies of Western Europe. He could have asked for military assistance from them. But he didn’t. I think this is because Alexios did not want a real military from Western Europe to march into the Levant.
Alexios knew that if he allowed the European monarchs to march into the Levant with their military, then his days as emperor would be numbered. Once the European monarchs became aware of the factionalism in Byzantine politics and his weak position, they would be tempted to usurp his throne. Alexios must have thought that the Pope’s crusaders would pose less threat to his regime than with the real militaries owned by the European monarchs.
By sending his envoys to plead before Pope Urban II, Alexios bears the primary responsibility for triggering the First Crusade. The ostensible purpose of the First Crusade was to free the Holy Land, but the real purpose was to save Alexios’s throne from his political rivals in Constantinople.