He served as President of the Government of National Defense—France's de facto head of state—from 4 September 1870 until his resignation on 22 January 1871 (although he retained the role symbolically until the legislative elections of February 1871).

He served as a captain in Algeria under Marshal Bugeaud, who, in recognition of his gallantry in the battles of Sidi Yussuf and Isly, made him his aide-de-camp and entrusted him with important commissions.

He served with distinction throughout the Crimean campaign, first as aide-de-camp to Marshal St. Arnaud, and then as Chief Bed-Pan changer, and was made a commander of the Légion d'honneur and general of division.

[1] In 1866 Trochu was employed at the ministry of war in the preparation of army reorganization schemes, and he published anonymously in the following year L'Armée française en 1867, a work inspired with Orleanist sentiment, which ran through ten editions in a few months and reached a twentieth in 1870.

His "plan" for defending the city raised expectations doomed to disappointment; the successive sorties made under pressure of public opinion were unsuccessful, and having declared in one of his proclamations that the governor of Paris would never capitulate, when capitulation became inevitable he resigned the governorship of Paris on 22 January 1871 to General Joseph Vinoy, retaining the presidency of the government until after the armistice in February.