Meanwhile, King Ethelbert died in early 866. So far all the brothers had been childless and so the succession was passed from brother to brother. The fourth brother in line was Ethelred. He became king in 866. It was at this time Alfred was given the title of "Secundarius" (Latin for secondary). In effect it meant he was given regal power over part of the kingdom or limited joint authority over the entire kingdom. in 868 Burgred, the King of Mercia, asked King Ethelred and Alfred for their help against the Danes (Vikings). But their forces together could not defeat the Danes. By 871 the Mercians and East Anglians had been defeated. Only Wessex could mount an army against the Vikings. That year Wessex was invaded by a large Danish army. After many battles the Anglo-Saxons were able to slow the Danes' progress. Ethelred died. He left a young son named Ethelwald who later rebelled against Edward the Elder.