The correct answer is - He promised to revive Roman greatness.
Mussolini often used the Roman Empire as the direct ancestor of Italy, and he used it in order to unite the people, to create strong nationalistic core in the country, thus to manage to achieve his goals.
One of the promises of Mussolini towards the Italians was that he will lead them to the glory that the Roman Empire had, that Italy will expand and regain all of the territories that the Roman Empire had, and that it will make Italy the greatest country in the world just like the Roman Empire was in its time.
His intentions though never came true, having a very badly organized military, poorly equipped and trained, Italy didn't managed to gain any new territories, and lost pretty much all battles when it came to confront a strong nation.