Final answer:
Alfred of Wessex defeated the Great Heathen Army in 878 at the Battle of Edington, which led to the Treaty of Wedmore and the creation of the Danelaw.
Step-by-step explanation:
King Alfred of Wessex achieved a significant victory over the Great Heathen Army in 878, which marked a turning point in the resistance against the Dane invaders. Although the conflict against the Danes persisted beyond this year with various levels of intensity and territorial disputes, 878 was the year when Alfred defeated the Great Heathen Army at the Battle of Edington, resulting in the Treaty of Wedmore and the establishment of the Danelaw. This decisive victory played a crucial role in preventing the conquest of all of England and set the stage for Alfred's later efforts in unifying the country.