Aircraft would have been of even greater value on August 16th, 1870, at the Battle of Rezonville, where neither the French nor the Germans were aware of the other's movements. On the 14th a battle had been fought east of Metz which had resulted in the French retreat. On the morning of the 16th Moltke thought the French had retired west by the Metz-Verdun road and those to the north of it, and consequently he directed his left wing due west towards the Meuse to head off the French, sending his right army towards Rezonville to harass their rearguard. The French retreat, however, had been slow and two corps were still at Rezonville, while three corps and the reserve cavalry were within easy reach, some 130,000 men in all. At 9 in the morning the German 3rd Corps, unaided and far from support, attacked a position within reach of the whole French Army, believing it had to deal with a rearguard only. Bazaine, on the other hand, thinking that he was faced by the German main army, remained on the defensive, and lost the opportunity of defeating in detail first the 3rd and then the 10th German Corps. A few aeroplanes operating on a radius of 30 miles would have disclosed between daybreak and 10 a.m. the true position to either commander. Neither the German nor the French cavalry, though both were engaged, obtained any reliable information. The problem as to how far aircraft would reduce the value of cavalry was widely discussed before the war. It was seen that by day aircraft could obtain quicker and more accurate information, but that cavalry retained their power of night reconnaissance, of mobile offensive action and of pinning the enemy to his ground by fighting. This was found to be so during the retreat, when, in addition to the direct value of aircraft for long-distance reconnaissance, an indirect asset of great importance lay in the release of the cavalry for battle action in assistance of the infantry. The question has become more acute since the offensive action of aircraft against ground targets has developed, but although we must never forget the splendid work of the mounted arm during the Retreat from Mons, and in March, 1918, factors have arisen tending to make the use of cavalry a problem of extreme difficulty in European wars, and it is possible that, in addition to their reconnaissance functions, aircraft will supersede the shock tactics and delaying action of cavalry, though this may be modified if, the sabre being a thing of the past, cavalry are converted into mounted machine gunners. Air tactics and training were, therefore, chiefly studied from the point of view of reconnaissance. In addition to the possibility of being shot at by other aircraft, an important consideration was vulnerability from the ground. Before the war reconnaissances were carried out at heights varying from 2,000 to 6,000 feet, but it was generally considered that the aeroplane was safe from fire from the ground at heights above 3,000 feet.