Founder of the Danelaw
It is not known how Guthrum consolidated his rule as king over the other Danish chieftains of the Danelaw (Danish-ruled territory of England), but by 874 he was able to wage a war against Wessex and its King, Alfred.
In 875, the Danish forces, then under Guthrum and , divided, Halfdan's contingent returning north to Northumbria, while Guthrum's forces went to East Anglia, quartering themselves at Cambridge for the year.
By 876, Guthrum had acquired various parts of the kingdoms of Mercia and Northumbria and then turned his attention to acquiring Wessex, where his first confrontation with Alfred took place on the south coast. Guthrum sailed his army around Poole Harbour and linked up with another Viking army that was invading the area between the Frome and Piddle rivers which was ruled by Alfred. According to the historian Asser, Guthrum won his initial battle with Alfred, and he captured the castellum as well as the ancient square earthworks known as the Wareham, where there was a convent of nuns.
Alfred successfully brokered a peace settlement, but by 877 this peace was broken as Guthrum led his army raiding further into Wessex, thus forcing Alfred to confront him in a series of skirmishes that Guthrum continued to win. At Exeter, which Guthrum had also captured, Alfred made a peace treaty, with the result that Guthrum left Wessex to winter in Gloucester.