"The advisability of employing gas as a military weapon is a matter for consideration by the authorities concerned before a campaign begins. Once authorised, however, and assuming that weather conditions are favourable, gas may be expected to play a part in every action. . . . The different methods in which gas can be employed make it a weapon which can be used by all arms, thus _Artillery_ deal with gas shells, _Infantry_ with light mortar gas bombs, _Aircraft_ with aerial gas bombs, and _Engineers_ with all methods of use that call for special manipulation" ("Field Service Regulations," vol. ii. (1921)). Gas was introduced by the Germans during the _Second Battle of Ypres_ (April 22-May 18, 1915), and the numerous experiments and trials necessary before gas can be used, and the great preparations which have to be made for its manufacture, show that its employment was not the result of a desperate decision, but had been prepared for deliberately. During the _First Battle of the Somme_ (September 1-November 18, 1916) "the employment by the enemy of gas and liquid flame as weapons of offence compelled us not only to discover ways to protect our troops from their effects, but also to devise means to make use of the same instruments of destruction. . . . Since we have been compelled, in self-defence, to use similar methods, it is satisfactory to be able to record, on the evidence of prisoners, of documents captured, and of our own observation, that the enemy has suffered heavy casualties from our gas attacks, while the means of protection adopted by us {177} have proved thoroughly effective" (Sir D. Haig's Dispatches). SMOKE Smoke can be discharged from _Artillery_ shells, Artillery or _infantry_ mortar bombs, Infantry rifle grenades, smoke candles, _Aircraft_ bombs, _Engineers'_ stationary generators, or the exhaust pipe of _Tanks_. It is used to conceal movement for the purposes of surprise or for reducing casualties, and can be so employed as to impose night conditions on the enemy while one's own troops retain the natural visibility; but while the weight and direction of an intended blow may thus be hidden from the enemy a warning is given of the time of its delivery. It is possible, however, to mystify, as well as to surprise, the enemy by the use of smoke, and its strategical and tactical value will ensure its adoption in Modern Warfare. In the closing battles of the Great War "the use of smoke shells for covering the advance of our infantry and masking the enemy's positions was introduced and employed with increasing frequency and effect" (Sir D. Haig's Dispatches). {178} OPERATION ORDERS Combatant officers of every rank are required to issue orders of some kind or other, and orders for operations should always be committed to paper when circumstances permit. The object of an operation order is to bring about a course of action in accordance with the intentions of the commander, and with full co-operation between all units.