Viper's Blood by David Gilman
Provoked by the Dauphin's refusal to honour the terms of his father's surrender, Edward III has invaded France with the greatest army England has ever assembled. But the English lion's attempts to claw the French crown from its master are futile. After defeats at Crecy and Poitiers, the Dauphin will no longer meet the English in the field.
Mired down in costly sieges and facing a stalemate, Edward's great army is forced to argee a treaty. But peace comes at a price. The French request that Blackstone escort their King's daughter to Italy to see her married to one of the two brothers who rule Milan - the same brothers who killed Blackstone's family to revenge the defeats they suffered at his hand.
Blackstone, the French are certain, will never leave Milan alive...