Since the Great European War broke out, printing presses have been busy producing text books, handbooks and guides for soldiers. Military authorities and civilians alike have come to realise that this war has changed many of our old conceptions of strategy and that it has introduced conditions that are entirely unprecedented. New methods have had to be devised-sometimes on the field itself in the midst of the greatest difficulties-for meeting new and novel methods of warfare. Every deadly engine of destruction has called forth some new invention to cope with it. Soldiers have had to live and fight under conditions that to the lay mind, or to the mind of the military men of a few years ago, would seem to be impossible. It is reasonable to assume that the inventive genius of the world will be turned more and more in the direction of the problem of how to construct still more terrible machines of destruction.