But it is difficult in war to keep any development hidden for long. Somebody blundered on one side or the other ; perhaps a prisoner boasted or an instrument was captured, and the secret was out. The experts of either side had taken the new invention of the triode valve, as it had been developed up to that point by wireless engineers, and had evolved from it an instrument so sen sitive that it could read the traffic over an enemy's cables some thousands of yards away. The mysterious forewarnings vouchsafed were merely the enemy's operation orders intercepted word for word. The extent to which such eavesdropping was possible de pended on various technical factors, and it was much reduced by the use of metallic circuits instead of earth return cables, but henceforth either side could use its forward cable system only in the knowledge that enemy listeners were straining their ears to overhear. Wireless telegraphy suffered of course at all times under the same limitation, to a much greater degree.
The Last Phase; Open Warfare.—On the Western front from the end of 1914 till the spring of 1918 the struggle grew in inten sity but did not change its nature. The combatants fought where they stood; the line sagged and swayed as here or there a short advance was made at bitter cost ; but it did not break and the chief concern of signalling was with fire power, to enable commanders to direct their ever-growing masses of emplaced artillery. Then
came the German break-through by a supreme effort in March 1918—a feat of arms which was to spend itself within sight of victory and to be checked, held and at last turned back by the Western armies in their final advance to victory. The armies fought once more over the open ; signalling was again concerned with troops in movement, and the underground telephone systems, constructed at such cost through three years of war, had to be left behind. The very reliance which their possessors had learnt to place in them became itself a handicap; few of the original com batants survived ; those few had grown accustomed to the war of the trenches, and to most of those who fought in 1918 war without a lavish telephone system was a strange conception. Moreover, the armies which emerged from their trenches in 1918 were not those which had gone to ground three years beforehand; with more ar tillery and machine guns, with aerial co-operation, with tanks and armoured cars and with the new tactical methods which all these demanded, they could never again be satisfied with the 1914 scale of signal communication. Commanders were faced with the prob lem of how to co-ordinate movement, to direct fire power and to administer their formations in open warfare, unaided by the trench telephone system to which all had grown accustomed.