In January 1867, "Flandre" collided with the British merchantman "Brutus" in the Atlantic Ocean. "Brutus" sank with the loss of ten of her fourteen crew. Together with "Magnanime" and "Magenta", "Flandre" escorted the transports ferrying French troops home after the collapse of the Second French intervention in Mexico in 1867. Two years later she became the flagship of Rear Admiral () Charles de Dompierre d'Hornoy. When the Franco-Prussian War began on 19 July 1870 the French needed time to complete their mobilization. The ship was assigned to Vice Admiral () Édouard Bouët-Willaumez's squadron that was tasked to blockade German ports in the Heligoland Bight. It departed Cherbourg on 24 July and, failing to find any German ships, proceeded to Danish waters to wait for further instructions. Bouët-Williaumez was ordered on 2 August to split his forces with half, including "Flandre", proceeding into the Baltic Sea to blockade the Prussian ports there under his command and the others to return to the Bight. The strong Prussia coastal defenses prevented any attack by the French ships, but their presence severely inhibited German shipping. "Flandre" and the ironclad were relieved by the ironclads "Rochambeau" and in August and returned to Cherbourg where "Flandre" joined the ships blockading the Bight.