Boris I of Bulgaria
Pribina, the Slavic "dux" of Lower Pannonia, died fighting the Moravians in 861, which suggests that Carloman also had conceded this province to Rastislav. In response to the ongoing rebellion of his son and Rastislav, Louis the German negotiated a counteralliance with Boris I of Bulgaria. The king made it seem that he was leading a new campaign against Rastislav, but at the last moment he moved against Carloman, who thus had no choice but to surrender.