Complex sentence: On November 16, De Bono was promoted to Marshal of Italy ("Maresciallo d'Italia"), but Mussolini grew ever more impatient with the invasion's slow progress and, by December, De Bono was relieved of his command.
Split into simpler sentences: On November 16, De Bono was promoted to Marshal of Italy ("Maresciallo d'Italia"). But Mussolini continued to grow impatient with his slow progress and, by December, De Bono was relieved of his command.