Complex sentence: The treaty was lenient toward Austria (It is speculated that this was due to a desire by Otto von Bismarck to use Austria in his plans for German unification), which only lost Venetia, ceded to Napoleon III, who in turn ceded it to Italy.
Split into simpler sentences: The treaty was lenient toward the Austrian Empire; it is speculated that this was due to a desire by Otto von Bismarck to use Austria in his plans for German unification. Austria only lost Venetia, ceded to Napoleon III of France, who in turn ceded it to Italy.