What sequence of events led to Alfred the Great becoming king?
Alfred became king of Wessex after his older brother, Athered, died in 871.
Alfred was the third son of Athelwulf of Wessex, and therefore unlikely to be crowned king himself due to the two brothers who preceeded him. He was an active member of the Wesex military during the time his brothers were reigning and even though historical sources are sketchy, we know that Alfred was crowned after his older brother died at the Battle of Menton.
What was interesting about Alfred gaining the throne was that his older brother did have two sons when he died. However, it wasn't uncommon for positions of power to move down through military commanders rather than just those related to the preceeding monarch.