The study of French military justice between 1870 and 1928 highlights a threefold observation: firstly, that there was, for some, a constant political desire to bring military justice and ordinary justice closer together. Secondly, that it was difficult for the French legislator, in peacetime, to provide for a functional and effective military justice system in time of war, given the uncertainty of the nature of the future conflict and the inability to predict its course and final outcome. Finally, the lack of respect by the military high command and the government for the principles of justice in dramatic military circumstances. Hence the improvised application of military justice, in practice, during the Franco-Prussian war and the First World War. What was intended to be modern, humane, and protective before the war became frightening, summary, and brutal during war. This raises the question of what military justice should be in war, or at least how to ensure that the principles of justice are respected during war. These reflections were at the heart of the post-conflict reforms and of the new code of military justice of 1928, a major step towards the civilianisation of French military justice.