The two previous chapters described the imperfectly defined and understood command relationship during the first two years of the war, both in operations in the field and in administrative problems such as port facilities. It was the liaison service that had the task of easing relations and making the partnership function. What machinery was put in place to overcome the obstacles of different military methods and lack of a common language? This chapter will consider the mechanisms of liaison at both military and political levels that were to solve these problems. After a brief account of what is involved in liaison, two sections focus on the service as it evolved, mainly under Joffre's direction, in 1914–15 when Sir John French commanded the BEF, and then on the service as it operated during 1916 when the only joint – or, rather, joined – battle of the war was prosecuted on the Somme.

The word ‘liaison’ is French. It comes from the verb lier, to bind or tie together, and this indicates the meaning in a military context: ‘that contact or intercommunication maintained between elements of military forces to insure mutual understanding and unity of purpose and action’. A liaison officer should act as the ‘eyes, ears and mouth of his commander’. So the role of any liaison service is to communicate in such a way as to bind together the actions of one or more commanders and their armies, thus increasing effectiveness, hence success.