Menabrea belonged to the right centre, and until the events of 1859 he believed in the possibility of a compromise between the Vatican and the state. He was major-general and commander-in-chief of the engineers in the Lombard campaign of 1859. He superintended the siege works against Peschiera, was present at Palestro and Solferino, and repaired the fortifications of some of the northern fortresses. In 1860 he became lieutenant-general and conducted the siege of Gaeta. He was appointed senator and received the title of count.