In April 1859 war returned. Maestri enlisted as a doctor-surgeon with Garibaldi's "Cacciatori delle Alpi" brigade, a proto-partisan operation created to support the Sardinian-Pietmontese army in the struggle to liberate Northern Italy. He also did valuable work as an intermediary between Agostino Bertani, chief medical officer of the "Cacciatori delle Alpi" and Count Cavour (who was in effect serving as his own Minister for War in the government that he led in Turin). The purpose of Maestri's diplomacy was to ensure that the volunteer army doctors who were supporting the anti-Austrian struggle were fairly allocated between the "Royal Sardinian Army" and the "Cacciatori delle Alpi". Neither Cavour nor Garibaldi would have dared take on the army of the armies of the Austrian Empire without the military and diplomatic muscle provided by French support. The French dictator loved to take his allies and enemies by surprise, and when he unexpectedly concluded with the Austrian emperor on 11 July 1859 it meant, to the bitter disappointment of Garibaldi, Cavour and their supporters, that the fighting was over. It was time for diplomacy. Long before November 1859, when the Treaty of Zürich was concluded, it had become obvious to commentators (and to subsequent generations of historians), that the Austrians had lost out. Garibaldi and Cavour, and indeed Pietro Maestri had fought on the winning side. In September 1859 Pietro Maestri returned to Paris and resumed his life as an increasingly prominent commentator on the twin themes of politics and economics, primarily with respect to Italy.