The First World War (1914–1918) has often been seen by historians of the French Language as a dividing line, raising the question of the coincidence between language change and historical change. One of the questions, e. g., is if the First World War was the moment when the use of dialects entered a phase of serious decline, as the soldiers sent to the front lines had to adopt French as a vehicular language. Picturesque aspects were also developed around the war by a significant amount of literary works, often published during the war itself, and tending to promote clichés and stereotypes. But what are the facts? This paper, divided in four parts, examines the different issues that can be raised around the linguistic production of this period, taking in turns the point of view of the linguist, the historian, and the literary critic. The fourth part looks more closely at the written production of the soldiers (letters, postcards). Indeed, during the war a number of individuals (soldiers and their families) had to write in French, perhaps for the first time ever, but certainly for the first time on such a scale. Is there a linguistic specificity to those written productions? Did the level of literacy evolve? This paper attempts to show that the First World War bears a fascinating potential for socio-linguistic enquiries, as some of its language material has so far remained unexploited and may reveal fruitful case studies for some of today's most important issues in the linguistic field.