By the end of WW1 there were some 70,000 signallers. In 1918, at the Battle of Amiens, trench warfare was largely replaced by the birth of modern warfare. The extensive use of artillery produced a further demand for dedicated signal sections, including liaison with spotter aircraft.
From the earliest of times armies have used some form of signalling. The Greeks had the torch telegraph and the water telegraph, the Roman Army used coloured smoke as a means of communication