The French were unaware of German numerical superiority at the beginning of the battle as the German 2nd Army did not attack all at once. Treating the oncoming attacks as merely skirmishes, Frossard did not request additional support from other units. By the time he realized what kind of a force he was opposing, it was too late. Seriously flawed communications between Frossard and those in reserve under Bazaine slowed down so much that by the time the reserves received orders to move out to Spicheren, German soldiers from the 1st and 2nd armies had charged up the heights. Because the reserves had not arrived, Frossard erroneously believed that he was in grave danger of being outflanked as German soldiers under General von Glume were spotted in Forbach. Instead of continuing to defend the heights, by the close of battle after dusk he retreated to the south. The German casualties were relatively high due to the advance and the effectiveness of the chassepot rifle. They were quite startled in the morning when they had found out that their efforts were not in vain—Frossard had abandoned his position on the heights.

What factor were the French unaware of at the start of the battle?