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.
Towards the end of the eleventh century, the Popes realized that the threat of excommunication was not a significant deterrence for the monarchs, and they conceived the idea of having an army that would be under papal control and could be used to subdue the recalcitrant monarchs. The idea of crusades was conceived. In principle, the crusaders owed allegiance to the papal authority. But the situation was different in practice and the crusaders often obeyed the monarchs. Between the twelfth and fifteenth centuries crusades were called against several excommunicated Christian monarchs.