Once Celtic Briton resistance had been largely subdued, Anglo-Saxon rule crystallized into seven main regional power bases, the Heptarchy, with Angles dominant in the Midlands and north, and Saxons in the south. An independent Jutish kingdom survived on the Isle of Wight until late in the 7th century. Borders were fluid, with parts of the Heptarchy frequently subsumed under the rule of an aspiring Bretwalda, but rarely for long. Remote areas were able to nurse a high degree of independence within these kingdoms well into the 8th century. The Magonsaete, Wreocansaete and Hwicce along the borders of Celtic Wales were not fully incorporated into Mercia until the reign of Offa (757–96), while the Haestinga, shielded by the Pevensey marshes and Weald, preserved a mini-kingdom until conquest by Offa in 771. In the north, Bernicia and Deira were distinct kingdoms, until unification into Northumbria under King Oswald after the battle of Heavenfield (633).
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