By the year 600, most of what is now England had been conquered by invaders from the continent, including Angles, Saxons, and Jutes. Bernicia and Deira, the two Anglo-Saxon kingdoms in the north of England, were first united under a single ruler in about 605 when Æthelfrith, king of Bernicia, extended his rule over Deira. Over the course of the 7th century, the two kingdoms were sometimes ruled by a single king, and sometimes separately. The combined kingdom became known as the kingdom of Northumbria: it stretched from the River Humber in the south to the River Forth in the north.