On 17 March Haig and the British army commanders met and discussed the effect of the German retirement. The precedent of a German withdrawal to a prepared position followed by a counter-attack, which had occurred in 1914 was noted and that reserves, freed by the retirement, would give the Germans an opportunity to attack the flanks of the withdrawal area. Nivelle had already decided to use the French troops released by the shorter front to reinforce the line in Champagne. British preparations for the attack at Arras were to proceed, with a watch kept for a possible German attack in Flanders and preparations for the attack on Messines Ridge were to continue. The pursuit of the German army was to be made in the Fourth Army area with advanced guards covered by the cavalry and cyclists attached to each corps and the 5th Cavalry Division. Larger forces were not to move east of a line from the Canal du Nord to the Somme south of Péronne until roads, bridges and railways had been repaired. The boundary of the Fourth Army and French Third Army was set from south of Nesle, through Offroy to St Quentin. In the Fifth Army area from Bapaume to the north, the advance to the Hindenburg Line needed to be completed in time to conduct supporting operations for the Third Army attack, due at Arras in early April. All-arms columns of cavalry, infantry, artillery and engineers were organised to advance on the front of each division. The advanced guards of the 5th and 2nd Australian divisions had a detachment of the Australian Light Horse, a battery of 18-pounder field guns, part of an engineer field company, two infantry battalions and several machine-guns. The advance had fewer geographical obstacles than further south. On the left flank the country beyond was open and on the right the Germans made little effort to hold the ground west of , the ground inclining slightly to the north-east towards Bullecourt, away, with most of the rivers flowing in the direction of the British advance.