Great Moravia arose around 830 when Mojmír I unified the Slavic tribes settled north of the Danube and extended the Moravian supremacy over them. When Mojmír I endeavoured to secede from the supremacy of the king of East Francia in 846, King Louis the German deposed him and assisted Mojmír's nephew Rastislav (846–870) in acquiring the throne. The new monarch pursued an independent policy: after stopping a Frankish attack in 855, he also sought to weaken the influence of Frankish priests preaching in his realm. Duke Rastislav asked the Byzantine Emperor Michael III to send teachers who would interpret Christianity in the Slavic vernacular.