In the eleventh century began the age of intense disputes between the papal establishment in Rome and the monarchies of Christendom. The Popes started excommunicating the monarchs to force them to obey the tenets of religion and the papal directives on political issues.
Several major figures of the eleventh century were excommunicated by the Popes: Henry IV of Germany (excommunicated by Pope Alexander II), Philip I of France (excommunicated by Pope Urban II), King Harold of England (excommunicated by Pope Gregory VII), Alexios I Komnenos of the Byzantine Empire (excommunicated by Pope Gregory VII), the Norman Duke Robert Guiscard (excommunicated by Pope Gregory VII). When the monarchs made the appropriate conciliatory gestures, the Popes allowed them to return to the communion but when another dispute emerged, the monarchs were excommunicated again. This became a regular feature of Christendom in Western Europe and the Byzantine Empire.
Towards the end of the eleventh century, the Popes realized that the threat of excommunication was not a sufficient deterrence for the monarchs. They thought that a papal army could be used to subdue the recalcitrant monarchs. But how to create a papal army? Some popes believed that the crusades could be used to create papal armies. In principle, the crusaders owed allegiance to the papal authority. But in practice, the crusaders obeyed the monarchs. To drive a wedge between the crusaders and the monarchs, the popes started calling for crusades against the excommunicated monarchs.