How did the crusades happen?

The Byzantine emperor Alexius Comnenus wrote a letter to the council of Piacenza in 1095 asking for help to free the christians in Jerusalem from persucation of the Muslims. The pope, Urban II gave permission to kill the infidels (the muslims) at the council of clermont and this caused this mass expedition which followed into the capture of Jerusalem. This first expedition was the Bowling ball for all of the other crusades that have followed.