Final answer:
Pope Urban II called for the First Crusade in France, rather than Rome, due to a rival pope's occupation of Rome, and the call was aimed at consolidating papal influence and uniting Christians. The goal was to liberate the Holy Land, which was imbued with great religious significance, thus resonating deeply with European Christians.
Step-by-step explanation:
Pope Urban II called for the First Crusade at the Council of Clermont in France, in 1095, due to the political circumstance of having a rival pope supported by the German emperor occupying Rome. Urban's call to arms against 'Turkic' control over the Holy Land aimed to defend Christians in those regions and did not initially advocate violence against Muslims broadly. The motive to liberate Jerusalem resonated more with Europeans than aiding Constantinople because of Jerusalem's religious significance as the life setting of Jesus and a place ingrained in Christian imagination as a 'gateway to heaven'.
The crusading directive also aligned with his efforts to reform the church and to consolidate papal influence by testing the support of common Christians for a religious cause. Urban leveraged the rhetoric of holy war, which utilized the perceived sanctity and pilgrimage aspect to inspire Europeans to participate.
Furthermore, this rallying was a strategic move by Urban to strengthen the church's frontiers, as he advised Spanish Christians to remain and combat Islam on the Iberian Peninsula rather than join the crusade. Over time, the difficulty in maintaining crusades due to the rise of powerful Islamic kingdoms like the Mamluks and Ottoman Turks, and shifting European aristocratic focus toward nation-state development, eventually led to the decline in crusading.