In 616, as a result of fighting the Battle of the River Idle and defeating Æthelfrith of Northumbria, he was able to install Edwin, who was acquiescent to his authority, as the new king of Northumbria. During the battle, both Æthelfrith and Rædwald's son, Rægenhere, were killed.

From around 616 Rædwald was the most powerful of the English kings