King Lothair II of Lotharingia will pressure Pope Nicholas I to grant him an annulment so he can marry his mistress, but the pope refuses.
A church council gives its approval for the annulment, at which point Nicholas deposes the council, excommunicates its messengers, and voids its decisions. The family of King Lothair II lays siege to Rome, but Nicholas stands firm and the annulment is never given.
Pope Nicholas I also tries to assert the authority of Rome over churches in the East, but this isn't as successful. As a result, he and Photius, patriarch of Constantinople, excommunicate each other. This will play an important role in the eventual schism bewteen Eastern and Western Christianity, known as the Great Schism.