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In page Halfdan Ragnarsson:

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The Army retreated to the captured town of London and stayed there over the winter of 871/872.[19] Coins minted in London during this period bear the name Halfdan, identifying him as the leader of the army.[17] In autumn of 872 the Army returned to Northumbria to quell a revolt against Ecgberht, their puppet-king.[18] However, this explanation for the army's move north has been challenged, and it has been suggested the relocation was a result of a war with Mercia.[20] In any case, the Army overwintered at Torksey, and then at Repton a year later. The Army finally conquered Mercia in 874, and their king, Burghred, was deposed and replaced by a puppet, Ceolwulf.[21]