The enemy referred to was Wedell's Brigade (38th), which, while on the march in the direction of Etain according to its original orders, had received counter-instructions while halted at St. Hilaire at noon, to hurry to the field of battle. General von Schwartzkoppen (commanding 19th Infantry Division) decided to march by the highway to Mars la Tour, in the hope of falling on the enemy either in flank or in rear. But the French meanwhile had extended their reinforced right wing to the sunken valley west of Bruville, where three Divisions of their cavalry were massed in position.
Thus when General von Wedell advanced to the attack on both sides of Tronville, which the French themselves had fired, his brigade--only five battalions strong--found itself in face of the long deployed front of the 4th French Corps. The two Westphalian regiments advanced steadily under the storm of shell and mitrailleuse fire till they suddenly reached the edge of a deep ravine hitherto unseen. This, however, they soon traversed, and were climbing the farther ascent, when they were met by a murderous shower of bullets from the French infantry which hemmed them in closely on every side. After almost every one of the commanders and regimental officers had fallen, the wreck of the battalions fell back into the ravine; 300 men were taken prisoners, having no strength left to ascend the steep southern rise after the fatigue of a twenty-eight miles march. The remainder rallied at Tronville under the shot-torn colours which Colonel von Cranach, the only officer who still had a horse under him, had brought back in his own hand. Seventy-two officers and 2542 men were missing out of 95 officers and 4546 men--more than half. The French followed up their success, but were checked on the right by the headlong charge of the 1st Guard Dragoons, which cost that regiment 250 horses and nearly all its officers; and on the left by the 4th squadron of the 2nd Guard Dragoons, which attacked three times its strength of Chasseurs d'Afrique.
But there now imminently threatened the charge of a great mass of French cavalry, which disclosed itself on the open plateau of Ville sur Yron. This consisted of Legrand's Division and de France's Guard Brigade in four compact echelons, overlapping each other to the right. On the German side, all the still disposable cavalry joined Barby's brigade, and the body thus made up, consisting only of sixteen squadrons, was formed for action in two lines west of Mars la Tour. Farther in advance stood the 13th Dragoons, halted to receive the Guard-squadron on its return from its recent charge. The 13th galloped forward to meet the charge of Montaigu's Hussar Brigade, which constituted the first line of the French cavalry mass, and which broke through the (over-wide) intervals of the Prussian squadrons. But General von Barby promptly appeared with the other regiments on the upland of Ville sur Yron, where at a quarter to seven the cavalry masses came into collision.