What was Byzantium doing? According to Geoffrey Hindley, Emperor Alexius needed reinforcements to accomplish a final push against the Seljuk Turks, and he went to the Franks for this. Before seeing the Franks, however, he wanted to get papal support, so his envoys went to Piacenza and met with Urban II. Urban asked the envoys to make a speech to the bishops and clerics who were there, and in this speech they played up the horrors of the Infidel. [Hindley, Geoffrey, A Brief History of the Crusades: Islam and Christianity in the Struggle for World Supremacy, Rev. pbk edn (London: Robinson, 2004), pp. 10-11.]
What about papal precedents? According to I.S. Robinson, Gregory VII 'recruited the "vassals of St Peter" by promising them absolution from their sins in return for their military service.' [Robinson, I. S., 'Gregory VII and the Soldiers of Christ', History: The Journal of the Historical Association, 58 (1973), 169-92, p. 180.]
Also, as i noted in my paper on the popes leading up to Gregory VII, Leo IX is quoted by his biographer as saying,
I considered it necessary to raise a defensive force from wherever men could be recruited to bear witness to [the Normans’] iniquity and, if it was expedient, to curb their arrogance…. Those who do not dread the judgements of heaven might at least come to their senses through the fear of men. [Anonymous, 'The Life of Pope Leo IX', in The Papal Reform of the Eleventh Century: Lives of Pope Leo IX and Pope Gregory VII, trans. by Robinson, I. S., ed. by Robinson, I. S. (Manchester: Manchester University Press, 2004), pp. 149-50.]
Unfortunately for the pope, not only were the Normans not afraid of ‘the judgements of heaven’, they were also not afraid of Leo’s army. His men were quickly beaten, and Leo was taken prisoner. Leo’s biographer notes, however, that the souls of Leo’s knights killed in battle were ‘united in heavenly glory with the holy martyrs’[Anonymous, 'The Life of Pope Leo IX', p. 151], a precursor to the indulgences allowed knights on crusade. This is the important precedent here: By calling together an army of men from wherever he could find them and sending them on a mission to save Christian souls, Leo created a prototypical crusade that could serve as the foundation for Urban II’s appeals in 1095.