He superintended the siege works against Peschiera, was present at Palestro and Solferino, and repaired the fortifications of some of the northern fortresses.

In October 1867, he succeeded Rattazzi in the premiership, and was called upon to deal with the difficult situation created by Garibaldi's invasion of the Papal States and by the catastrophe of Mentana.

[1] Menabrea disavowed Garibaldi and instituted judicial proceedings against him; but in negotiations with the French government he protested against the retention of the temporal power by the pope and insisted on the Italian right of interference in Rome.

He was in on the secret of the direct negotiations between Victor Emmanuel and Napoleon III in June 1869, and refused to entertain the idea of a French alliance unless Italy were allowed to occupy the Papal States, and, on occasion, Rome itself.

His successor in the premiership, Giovanni Lanza, in order to remove him from his influential position as aide-de-camp to the king, sent him to London as ambassador, where he remained until in 1882 he replaced General Cialdini at the Paris Embassy.