In the beginning of the present war, after a few weeks of open conflict, this kind of fighting in France gave way to a vast siege. The English found themselves, after some cross-country fighting, in the trenches. There the operation order which had provided for situations of march, halt, bivouac, and battle, was gradually made over to suit the peculiar needs of defenses, raids, and frontal attacks. The barrage fire, bombs, mortars, machine guns, and intricate lines of communication added to the number of details to be considered. Little by little the will of the commander expressed itself in such completeness that the former size of his operation order passed all bounds. It was natural, then, that although the scope and character of the English order steadily changed, its name remained the same. When the Americans came to take part in the war, the operation order had during the previous years of struggle developed out of all resemblance to its former self. It was no longer a field order as we know the term. It was fully grown to meet the tremendous progress of English experience in the trenches. The consequence was that the American, finding both the state of siege and the order to cover it beyond his experience, associated the English name with the novel warfare. Thus, today, the operation order is a term used in connection with the disposition of troops on the battle fronts of Europe. We should bear in mind, then, that the operation order is nothing more than a field order fitted to trench conditions. We must not gain the idea that it has supplanted the order which goes with mobile exercises on open ground. Open warfare began this conflict, and will, provided it be ended by martial victory, bring it to a close. Open warfare has been the rule on the long Russian, Italian, Serbian, Turkish, and Roumanian fronts. We must not be led astray in our estimate of the proportions because the front in France and Belgium is the most vital one to us from a strategic standpoint. Field orders, which are the accompaniments of open warfare, should still form the larger part of our study. Nevertheless, the operation order is the order next in importance to the field order. It must be framed with mathematical precision under fire. It has the greatest conciseness, but is not brief. It goes into the minutest details, but its details are tersely put. In certain parts it is telegraphic like the field message; in others it has completed sentences like those of the field order. It has an orderly arrangement of paragraphs and brief and unmistakable language. If we are able to compose a good field message and order, we should find, after we have familiarized ourselves with the technique of the trenches, that the construction of the operation order will fall naturally into place in our minds.