Book
The Lily and the Lion
Book 6 in the The Accursed Kings series.
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Charles IV dies without a surviving son, ending the direct Capetian line. Robert of Artois helps Philip of Valois secure the crown as Philip VI rather than allow Edward III's claim through Isabella of France, expecting the new king to reward him by overturning Mahaut's possession of Artois.
Robert's use of forged evidence destroys his position at the French court and drives him into exile in England. There he encourages Edward III's claim to France, turning a private feud into one of the political forces behind the opening of the Hundred Years' War.