Why was the interdict effective weapon for the pope to use against the king?

The interdict was an effective weapon for the pope against a king because it suspended religious services and sacraments in the affected region, causing widespread distress among the populace. This created pressure on the king, as the people often blamed him for their spiritual suffering and sought to hold him accountable. The interdict undermined the king’s authority and legitimacy, forcing him to negotiate with the pope to lift the suspension and restore normalcy. This dynamic showcased the pope's significant influence over both spiritual and temporal matters during the medieval period.