He fought in the Franco-Prussian War, at first as Chief of staff under general Laveaucoupet, commander of the 3rd Infantry Division integrated into 2nd Army Corps of general Frossard. Billot participated in the battles of Sarrebruck, Forbach where he was mentioned in dispatches, Borny and Noiseville. He managed to escape after the capture of Metz and put himself in the service of the Government of National Defence. He was promoted to colonel, then général de brigade and provisional général de division. Chief of staff then commander of the 18th Army Corps, Billot was beaten at Beaune-la-Rolande on 28 November 1870. Confirmed as permanent général de brigade, he fought in the battle of Villersexel in January 1871.