The 9th century was a harsh time for Northumberland. The Vikings were at their most active at this time. Several bands of pagan raiders came over the North Sea to loot and pillage the countryside, combining into large armies whenever resistance materialised, then scattering again to steal everything they could when no English armies were nearby.
In 867 Ivar the Boneless, the leading Viking leader, crushed a Northumbrian army, killed the English king and set up an obscure nobleman named Egbert to be the puppet ruler of Northumbria. In 873 Egbert died, whereupon the Northumbrian nobles elected a man named Ricsige and declared that no more tribute would be paid to the Vikings. Ivar was by this date dead, his place taken by his brother Halfdan who was busy fighting in southern England. Once a truce had been arranged, and tribute gained, in the south, Halfdan ordered the Viking army back north to crush the Northumbrians. Half the Viking army refused to obey, preferring to pillage East Anglia instead. Nevertheless, Halfdan arrived in the Tyne in the spring of 875 with a sizeable army, perhaps some 10,000 strong.
King Ricsige decided against facing the fearsome Vikings in open battle, preferring instead to wage a war of raid, ambush and skirmish. His key aim appears to have been to deny either food or money to the Vikings. Ricsige knew the Vikings were after loot and hoped that they would not stay in Northumbria long if they did not get much of it. Food was just as important to a raiding Viking force, so Ricsige ordered his subjects to hide all food stores and drive their cattle into the dense forests and high mountains of Northumberland.
Halfdan countered by dividing his force and sending them deep into the Northumberland interior with orders to find and kill Ricsige, while gathering as much food and loot as possible. Sometime in the summer of 875, the date has been lost, one force of Vikings rowed their ships up the coast to the Aln, beached their ships at Alnmouth and marched up the river.
The details of the subsequent battle have been lost for the simple fact that almost nobody survived the slaughter to tell the tale. We do not even know the names of the rival commanders. Nevertheless, the outline of events can be traced fairly accurately.