The character of Viking attacks on England decisively changed in the year that Æthelred succeeded to the throne. Previously the country had suffered from sporadic raids, but now it faced invasion aiming at conquest and settlement. A large force of Vikings, called by contemporaries the Great Heathen Army, arrived in East Anglia. King Edmund purchased peace by paying tribute and the Vikings stayed a year building up their strength. They then marched on York and conquered Northumbria, installing a puppet king. In late 867 they took Nottingham in Mercia and spent the winter there. Æthelred's brother-in-law, King Burgred, appealed to him for help. Æthelred and Alfred led a large West Saxon army to Nottingham and besieged the Vikings, but they refused to leave the safety of the town's defences. The combined Mercian and West Saxon armies were unable to breach the earth ramparts and ditch, and eventually Burgred bought them off. The Vikings then went back to York.