In 1866, in anticipation of the war with the Austrian Empire, after the signing of the Italo-Prussian alliance, Ricasoli was appointed Prime Minister by the king to replace General Alfonso Ferrero La Marmora, who would lead the Italian troops during the war. The king wanted to established a government of "national conciliation" that included members both form the Right and the Left. On 20 June, Depretis was appointed Minister of the Navy, on the same day the Third Italian War of Independence began. After the Italian defeat in the Battle of Custoza, Depretis insisted with admiral Carlo Persano on the attack against the island of Lissa, as a revenge for Custoza. But he also refused to give to admiral Persano detailed orders about the expedition in the Adriatic Sea against the fleet led by Wilhelm von Tegetthoff. However the Italian Royal Navy was soundly defeated. To quell the public outcry after the two defeats, Depretis called for process Persano, who was judged by the Italian Senate, condemned for incompetence in 1867 and cashiered from duty. However the war ended in an Austrian defeat, with Austria conceding the region of Venetia to Italy. Italy's acquisition of this wealthy and populous territory represented a major step in the process of Italian unification. On 17 February 1867, Depretis resigned as Minister of the Navy. His tenure as minister was quite controversial, his apologists contend, however, that, as an inexperienced civilian, he could not have made sudden changes in naval arrangements without disorganizing the fleet, and that in view of the impending hostilities he was obliged to accept the dispositions of his predecessors. Leader of the Left On 17 February 1867 Depretis was appointed Minister of Finance in the second cabinet of Ricasoli; however after few months, on 10 April, Ricasoli resigned and Depretis returned to be a simple deputy. Upon the death of Rattazzi in 1873, Depretis became leader of the Left. On 25 June 1873, the conservative government of Giovanni Lanza fell, beaten by a vote that saw the Depretis's moderate Left united with a large part of the Right, no longer in support of the Minister of Finance Quintino Sella; in fact Sella abandoned the proposal of a further fiscal tightening in order to achieve a balanced budget. Victor Emmanuel II then asked the conservative Marco Minghetti to form a new government, inviting him to include also members from the Left.