The Life of Charlemagne

Then the Bavarian war broke out suddenly, and was swiftly ended. It was caused by the pride and folly of Tassilo, Duke of Bavaria; for upon the instigation of his wife, who thought that she might revenge through her husband the banishment of her father Desiderius, King of the Lombards, he made an alliance with the Huns, the eastern neighbours of the Bavarians, and not only refused obedience to King Charles but even dared to challenge him in war. The high courage of the King could not bear his overweening insolence.