Pictish recorded history begins in the Early Middle Ages. At that time, the Gaels of Dál Riata controlled what is now Argyll, as part of a kingdom straddling the sea between Britain and Ireland. The Angles of Bernicia, which merged with Deira to form Northumbria, overwhelmed the adjacent British kingdoms, and for much of the 7th century Northumbria was the most powerful kingdom in Britain. The Picts were probably tributary to Northumbria until the reign of Bridei mac Beli, when, in 685, the Anglians suffered a defeat at the Battle of Dun Nechtain that halted their northward expansion. The Northumbrians continued to dominate southern Scotland for the remainder of the Pictish period.