A variable is what you are trying to measure in your study. The dependent one is the one that is being tested. This is being changed by the effect of the independent variable, its behavior is ruled but certain conditions that can change, because mainly the effect of those change is why the experiment was made.
The Christians want to win back the holy land and the pope wishes to see the holy land under Christian rule so they set up the crusade. There was also a King's Crusade which the pope set up.