The French held considerable forces in the immediate vicinity to exploit successes. It was reckoned that the enemy could not reinforce his front under forty-eight hours. Everything depended in the first instance on successful surprise, and in the second on securing within forty-eight hours the important tactical points within the salient. The difficulties of surprise, which were many and serious, were most successfully overcome, but the enterprise failed eventually because the key points were not seized. The principal factors operating against success were the limited hours of daylight and the long distances to be traversed both by men and by tanks, which, though vastly improved since 1916, were still very slow. There was also, in the case of securing the high ground west of Cambrai, the canal to be crossed by tanks. While smashing in the enemy's salient we ourselves were making a salient, extending our front, as far as the Third Army was concerned, from a straight 7,000 yards to a curving 15,000 yards, thus affording the enemy a chance of a blow at the sides and hinges of the salient, of which he availed himself to good purpose ten days after our initial attack. To ensure success the troops which were to undertake operations practised with tanks in back areas, and officers and men went through the operation on a carefully made ground model without being aware what ground it represented. Units were brought up just before the 20th of November, the day of the attack, marching by night and hiding in villages and woods by day. In some cases battalions were quartered in flat canvas erections, looking like ammunition or supply dumps. The 6th Division were fortunate in being in woods and destroyed villages. No unusual activity on ground or in the air was allowed, no guns registered as had been usual, even the Home mails were stopped for a short period, and a screen of the troops which had held the line for some time was kept in front trenches to the last. Under General Byng's initiative the difficulty of tanks crossing the wide Hindenburg Line trenches was overcome by each tank carrying on its brow a huge faggot which it deposited in the trench at its selected crossing-place, and which gave its tail a purchase to enable it to climb the opposite side of the trench. The ground was very suitable for tanks, as it was moderately hard grass land, and the first portion of the attack on much of the front was downhill.