Appointed commander of Belfort in 1870, Colonel Denfert-Rochereau was confronted from November 1870 with the attack and the subsequent siege of the city by the German armies under August von Werder.

When asked to surrender the fortress, the Colonel responded: "We are aware of our duty towards France and the Republic, and are resolved to respect it".

The heroic resistance would eventually end only under the order of the Government of National Defense on the 18 February 1871: Denfert-Rochereau accepted to leave the stronghold with his troops and arms, freely and unconquered, avoiding the humiliation of a defeat.

Refusing the armistice, he urged (de facto) President Thiers to pardon the young officer Louis-Nathaniel Rossel—also a Protestant— who had joined the Paris Commune after the French defeat.

The resistance of Denfert-Rochereau saved the honour of France, otherwise humiliated by MacMahon's defeat at Sedan and Bazaine' surrender at Metz.