Royal Engineers
“All arms are responsible for the construction of their own works of defence. It is the duty of the Royal Engineers to assist them by engineer reconnaissances, plans, advice, technical supervision, provision of materials and the construction of works requiring special technical skill.... Although trained as fighting troops, engineers should be regarded as reserves to be used only as a last resource; casualties in their ranks are not easy to replace, and they may become needlessly involved in the fighting and lost for work which may have an important bearing on the operations” (“Field Service Regulations,” vol. ii. (1921)).