In July 1870 the Franco-Prussian War started which led to the fall of the Second French Empire of Napoleon III. On the establishment of the Third Republic in September 1870, he offered his services to Léon Gambetta, was appointed prefect of the department of Tarn-et-Garonne, and in October became chief of the military cabinet. It was mainly Freycinet's powers of organization which enabled Gambetta to raise army after army to oppose the invading Germans. He revealed himself to be a competent strategist, but the policy of dictating operations to the generals in the field was not attended with happy results. The friction between him and General d'Aurelle de Paladines resulted in the loss of the advantage temporarily gained at Coulmiers and Orléans, and he was responsible for the campaign in the east, which ended in the destruction of the Armée de l'Est of Charles Denis Bourbaki.