After the defeat of the Kościuszko Uprising, Joselewicz left for Galicia and then for Italy. There he joined the Polish Legions under Henryk Dąbrowski. As a commander of a sabre company in Polish cavalry units, he fought in various battles of the Napoleonic Wars period. Among them were the battles of Trebbia, Novi, Hohenlinden, Austerlitz and Friedland.