In the Battle of Najera, the
Black Prince
and his brother
John of Gaunt united forces
with
Pedro of Castile in an armed combat
against Pedro's brother Henry of Trastamara. Henry and his allies were defeated.
This image is from a fifteenth-century manuscript of Jean Froissart's
Chronicles
(Bib. Nat. Fr., FR 2643, fol. 312v).