Lothar II (Louis II's brother) wanted to divorce his wife, Theutberga, and marry his mistress. The local archbishops immediately granted him a divorce. Nicholas sent papal legates to investigate. Lothar bribed the legates. Nicholas found out about it and ordered Lothar to take back his wife or be excommunicated. Nicholas said, "It is not coitus that makes marriage but Intention." Lothar marched against Rome. Nicholas fasted and prayed for two days in St. Peter's Basilica until Lothar backed down.
Nicholas was also involved with the schism between the Greek Church, based in Constantinople, and his own Church of Rome. The Byzantine Empress, Theodora, had elevated Ignatius to Patriarch of Constantinople, the head of the Greek Church. At the time the Prime Minister, Caesar Bardas, had divorced his wife and was living with his son's widow -- technically a case of incest. Ignatius denounced Bardas for this.
In 858, Bardas banished Ignatius from the Byzantine Empire and raised Photius to be Patriarch. Photius was a master scholar who had written the Myriobiblion, a sort of encyclopedia. Meanwhile, Ignatius went to Rome. Nicholas said that the Eastern Church had exceeded its authority and excommunicated Photius. Bardas threatened military intervention.
In 863, Nicholas replied:
"We have not invaded Crete;We have not subdued Greece;Wehave not burned the churches in the very suburbs of Constantinople; yet while these pagans with impunity conquer, burn, and lay waste [your territories], we, Catholic Christians, are menaced with the vain terror of your arms. Ye release Barabbas, and kill Christ!"
Nicholas continued to meddle in the affairs of the Eastern Church. In 866, a number of Bulgarian pagans had been converted to Catholicism and sent for a bishop -- not to Constantinople, but to Rome. They asked the Pope to send a "judgement of penance" -- a sort of scale for determining the penance for certain sins.
Nicholas replied:
"The Judgement of Penance for which you ask, our bishops whom we have sent into your country will indeed bring with them among the documents, or assuredly the bishop who will ordained among you will exhibit it at a suitable time; for it is not proper for secular persons to have any such thing, for in fact no ministry of judgement by this means is accorded to them." (Italics added)
When Nicholas died in 867, Photius was de-excommunicated and reestablished as the Patriarch of Constantinople. The Eastern and Western Churches remained allies for several more centuries.