"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 aërial 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).