In April 1866 La Marmora concluded an alliance with Prussia against Austria-Hungary, and, on the outbreak of the Third Italian War of Independence in June, took command of an army corps. He is largely credited of the hesitant conduct of the first phases of the Italian invasion, which, despite the large Italian superiority, led to the defeat in the battle of Custoza on 23 June. Accused of treason by his fellow countrymen, in particular by other high rank generals, and of duplicity by the Prussians, he eventually published in defence of his tactics (1873) a series of documents entitled "Un po' più di luce sugli eventi dell'anno 1866" ("More light on the events of 1866") a step which caused irritation in Germany, and exposed him to the charge of having violated state secrets.