This thesis examines how the Duke of Wellington used Portuguese and Spanish forces during his 1812 Salamanca campaign. Wellington assessed the strengths and weaknesses of his allies, and then leveraged them throughout the campaign within the constraints of dissimilar command relationships. He was able to supplement his British formations largely with Portuguese forces, as well as and prevent the numerically superior French from massing on his army through influence and interaction with Spanish forces. Scrutinizing how Wellington engaged in military actions with allies who had divergent political interests and varying degrees of military capability offers lessons in coalition warfare that are still applicable today.