Previous scholarship on the Allied coalition during the First World war has tended to stress the misunderstandings and distrust between the individuals representing their countries at high command level. There is an unexplained tension between this mésentente and the durable nature of the coalition and eventual victory of the French, British and Americans on the Western Front which leaves the lower echelons under-explored. By dialectically crossing communication practice and military logistics we can write a history which tells us both how these exchanges were possible and to what extent they contributed to the Allies’ victory. Official and private archival material enables us both to read traces of language from the perspective of the history of international exchanges and also to understand choices in military logistics from the point of view of interpreting and translation studies. France and Britain had very different military language strategies throughout the 19th century, with the French developing a specialised corps of military interpreters recruited by competitive exams while the British army relied on the one hand on officers qualified in foreign languages and on the other on guides and scouts recruited locally. Secret negotiations between 1905 and 1912 establied a strategy consisting of attaching English-speaking French soldiers to British units. From August 1914 these untrained military interpreters were put to the test and had to learn to position themselves among the British officers, French authorities and the local civilian population behind the lines. Battlefield liaison and the handling of the paper bureaucracy of modern warfare were two further important elements in this fragile but successful balancing act which was further challenged in 1918 with the progressive integration of the US troops in the Allied operations and the shift from the trenches back to mobile warfare.