Little is known about how international humanitarian law has developed around the world, other than in Europe and the USA. However, it is a topic worth researching, as it may reveal new connections, causalities and the previously unknown origins of legal institutions. Mexico is a good example of how the rules of war developed differently in different countries, since – as early as 1871 – it incorporated the law of war in its domestic criminal law. This article will explore how the idea of criminalizing violations of the laws of war flourished in nineteenth-century Mexico. A combination of factors including foreign interventions, civil wars, the liberal convictions of the drafters of the Mexican Criminal Code and their will to achieve the rank of “civilized nations” led to the creation of the crime “violations of the duties of humanity”. This development was a milestone in the history of pursuing individual criminal responsibility for violations of the laws of war and, therefore, is a missing piece in its history.
