Complex sentence: Upon the death of Rattazzi in 1873, Depretis became leader of the Left, prepared the advent of his party to power, and was called upon to form the first cabinet of the Left in 1876.
Split into simpler sentences: Upon the death of Rattazzi in 1873, Depretis became leader of the Left. He prepared the advent of his party to power and was called upon to form the first cabinet of the Left in 1876.