This term had, long before, passed into the homely phraseology of the war, in order to describe a procedure by which one body of troops, having reached its objective, was there halted, as at a completed task, while a second body of troops, of similar order of importance, but under an entirely separate Commander, advanced over the ground won, reached the foremost battle line, took over the tactical responsibility for the fighting front, and after a prescribed interval of time continued the advance to a further and more distant objective.

This conception of an advance by a process of "leapfrog" had been evolved early in 1917 in connection with a method of assault on successive lines of trenches. It was intended at the outset to be applied only to very small bodies of infantry, such as platoons. A normal battle plan for a company of infantry of four platoons was for the first two platoons to capture and hold the front line trench, while the next two following platoons would leap over this trench and over the troops who had gained it, and then pass beyond to the capture of the second, or support trench. The method was used, for the first time, on such a modest scale, at the battle of Messines, in June, 1917, and later on in the same year was adopted for bodies as large even as Battalions, in the fighting for the Broodseinde and Passchendaele heights.

But on no previous occasion had such a principle been applied to whole Divisions. It is true that at the battle of Messines, the Fourth Australian Division passed through the New Zealand Division after the latter had completed the capture of the main Messines ridge, but this was really exploitation, undertaken in order to take advantage of the temporary confusion of the enemy, and for the purpose of gaining ground upon the eastern slopes of the captured ridge. It was not a movement which was really part of the main assault, and it was confined to a single Division.

On the present occasion my purpose was to carry out a clear and definite process of "leapfrogging," not only simultaneously by two Divisions side by side, but also as an essential part of the time-table programme for the main battle, and before the exploitation stage of the fighting was timed to be reached. It was, undeniably, a daring proposal, involving very definite risks, enormously increasing the labour of preparation and the mass of detailed precautions which had to be undertaken in order to obviate the possibility of great confusion.

The preparations necessary for a single Division proposing to advance alone, to a prescribed distance, over country much of which was usually visible to us from our front line, are sufficiently complex, relating as they do, not only to the establishment of numerous protected headquarters for Brigades and Battalions, of miles upon miles of buried and ground cables, of dumps of all kinds of supplies, and of dressing stations and medical aid posts; but also to the disposition, in concealed positions, of all the assaulting units, down to the smallest of them, of Infantry Engineers and Pioneers. All these preparations assume a tenfold complexity when a second Division has to make arrangements exactly similar in character, variety and extent, using exactly the same territory for the purpose and at the same time, and planning to advance over more distant country, entirely beyond visual range and preliminary reconnaissance.

The project also involved a much greater crowding of troops into the areas immediately behind our line of departure, and, therefore, enormously increased the risk of premature detection by the enemy, both from ground and from air observation, of unusual movement and of other symptoms which presaged the possibility of an attack by us. The plan also necessitated the closest possible co-ordination of effort, and mutual sympathy and understanding, between the Commanders and Staffs of the twin Divisions having a common jurisdiction over one and the same area of preparation, and one and the same battle front. This was a degree of co-operation which could not have been looked for unless the personnel concerned had already established, from long and close association with each other, the most cordial personal relations. And dominating all other difficulties were those involved in the proposal to execute this difficult and untried operation of a Divisional leapfrog, not singly but in a duplex manner, necessitating the assurance of exactly similar simultaneous action, similarly timed in every stage, both before and during battle, by each of two separate pairs of Divisions.

These threatening difficulties were surely formidable enough, but I knew that I could rely upon the goodwill of the Divisions towards each other, and upon the loyal support of them all. This seemed to me to justify the attempt, and to minimize the risks; having regard above all else to the results which I stood to gain if the operation could be executed as planned.

On no previous occasion in the war had an attempt ever been made to effect a penetration into the enemy's defences at the first blow, and on the first day, greater than a mile or two. Rarely had any previous set-piece attack succeeded in reaching the enemy's line of field-guns. The result had been that the bulk of his Artillery had been withdrawn at his leisure, and his losses had been confined to a few hundred acres of shattered territory. But the task I had set myself was not only to reach, at the first onslaught, the whole of the enemy's Artillery positions, but greatly to overrun them with a view to obliterating, by destruction or capture, the whole of his defensive organizations and the whole of the fighting resources which they contained, along the full extent of my Corps front.

To achieve this object I prepared my plans upon the basis of a total advance, on the first day, of not less than 9,000 yards. This was to be divided into three separate stages, as follows:

Phase A—Set-piece attack with barrage,3,000yards.
Phase B—Open-warfare advance,4,500"
Phase C—Exploitation,1,500"
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Total distance to final objective,9,000yards.
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