How did popes 11 Th century use excommunication and interdicts as political tools?

If a kingdom or country is under interdict, the priests are forbidden from performing their normal priestly duties. When a large population of Catholics finds that they can no longer attend Mass or receive the sacraments, they tend to get very upset with their leader who caused the loss.

If a leader, such as a king, is excommunicated, all oaths and promises sworn to the leader are rendered invalid. The leader loses control of his subjects.