Images can help us to better understand how the laws of war and international humanitarian law have developed. As images transmit several messages simultaneously, they can reveal contextual, ideological, political and emotional dimensions that legal texts may not fully capture. This article aims to be an exercise seeking to demonstrate how a painting can serve not only as a historical source but also as a trigger for curiosity, prompting deeper exploration into the past and into the history of the laws of war. The painting in question, an 1881 piece by the Mexican artist Francisco de Paula Mendoza entitled The Pardon of the Belgians or The Exchange of Belgian Prisoners, will transport us to a mid-nineteenth-century exchange of prisoners of war (PoWs) and will remind us of the long-forgotten chapter of the Belgian volunteer force in what is known as the French Intervention in Mexico (1862–67). Drawing on primary and secondary sources, the article will offer new perspectives on the status of PoWs, the practice of the exchange of PoWs, and the doctrinal underpinnings of these ideas, and will reveal the entanglements between the laws of war, the principle of equal sovereignty, self-determination, non-intervention and republicanism in nineteenth-century Latin America. It will also reveal the interest of Latin American republics like Mexico in formalizing and standardizing the practice of the laws of war.