Born of an extremely brief diplomatic crisis, the Franco-German war of 1870 sealed the fate of the Second Empire. In three days, from August 4 to 6, the imperial army suffered three major defeats during the battles of Wissembourg and Froeschwiller (Alsace) and the battle of Spicheren (Lorraine). These defeats opened the doors of France to German troops. Unlike the two battles fought in Alsace, the Battle of Spicheren, led on the French side by Bazaine and Frossard, should have been a French victory. Indeed at the start of the Prussian offensive, and for several hours, a single enemy division faced three French divisions. What are the reasons for this defeat?