Boris I of Bulgaria
In 862, at the invitation of their ally the Moravian leader Rastislav, the Hungarians first raided Lower Pannonia. The following year, Louis the German, king of Eastern Francia, retaliated by forging an alliance with the Bulgarians. Boris I of Bulgaria sent mounted troops to help defeat Rastislav. This retaliation began an ongoing conflict which lasted for 25 years, pitting Hungarians and Moravians against Bulgarians and Franks.