On July 13 the Fourth Army Commander was asked by G.H.Q. to submit a scheme for an attack on his front. This was done on the 17th, when a limited operation, with the object of capturing the Amiens outer defence line, running from Castel through Caix to Mericourt, was outlined. The force suggested for this attack was three corps and eight battalions of tanks. On the 21st a conference was held at the Fourth Army headquarters at Flixecourt when, on the suggestion of the Tank Corps, the number of tank battalions was raised from eight to twelve; this comprised the whole Tank Corps less the 1st Tank Brigade, which was still armed with Mark IV machines, and which at this time was engaged in training its personnel on the Mark V tank.
On July 27, zero day was fixed for the 10th, but on August 6 this was changed to the 8th. All this time, in order to maintain secrecy, no mention of the impending attack was permitted, and the only preparation which could be undertaken was to send one officer of the Tank Corps General Staff to the area of operations to study the ground. On July 30 a conference was held at the 5th Tank Brigade headquarters at Vaux, at which the Fourth Army Commander explained the plan of operations. From this day on preparations were begun, the railway moves being issued the same evening.
As already stated, the original proposal was a limited operation, the centre of the attack being carried out by the Canadian and Australian Corps. The right of the Canadians was to be covered by the French First Army attacking east and south-east of the Luce river. The left of the Australians was to be protected by two divisions of the IIIrd Corps operating towards Bray. On July 29 the scope of the operation was extended as follows:
To disengage the Amiens-Paris railway by occupying the line Hangest--Harbonnières--Mericourt.
To advance to the line Roye--Chaulnes, driving the enemy towards Ham, and so facilitate the advance of the French on the line Noyon-Montdidier.
The force placed at the disposal of the Fourth Army consisted of the following Corps:
(iv) General Reserve--3 divisions, to be supplemented by further divisions as soon as possible.
(v) The Cavalry Corps--3 cavalry divisions.