Like everyone else, dictators have a historical context, a place in time where they learned to be the autocrats they eventually became. Sometimes they learned from the real thing, other times from pretenders. Think Julius Caesar learning from Sulla's example, Boy Assad learning from his, ahem, father, or Hitler watching the anti-Semitic Vienna mayor Karl Lueger operate.
Mussolini had an antecedent too, a very close one. But Gabrielle D'Annunzio wasn't just a would-be tyrant; he was more, a poet, novelist, lover, aviator, orator, and all-around son-of-a-bitch.
He certainly personified many of traits we listed in The Dictator's Handbook: megalomaniacal selfishness, enormous eccentricity, action-loving, bearer of a cool nickname ('the Commandant'), ladies man, lover of war and violence (rhetorical violence, in any case), spiffy uniforms, ... and hey, if you can take over a city for the hell of it, why not? Clearly, Mussolini was taking notes and in a few short years would follow D'Annunzio's example.
He's also the subject of an excellent new biography that certainly seems to highlight what made his example so crucial for many of the European dictators of the period between 1920 and 1940.
And he was indeed a pretty good and original writer. Very enjoyable is his collection of stories Little Tales from Pescara usually translated as Tales of My Native Town (see the story 'The Idolators,' for example).
Finally, for the last word on D'Annunzio, perhaps it's best to go to Ernest Hemingway's short, sardonic, poem that is titled after the proto-dictator himself.