As Lilo and PB noted, the Chicago Outfit under Torrio and Capone was not Cosa Nostra. The Mafia equivalent in Chicago at that time was the Unione Siciliana, a kind of civic group with muscle. Sicilians only. Capone constantly tried to get one of his own men of Sicilian extraction as their president.
Puzo based that part of the novel on the famous Castellemmarese War of 1930-31. He took liberties--for example, having Vito fight and beat "Maranzano" when in real life Salvatore Maranzano vanquished Joe the Boss Masseria. Messaria, not Maranzano, died in a restaurant ambush with his mouth full of half-chewed bread.
As for Capone: Masseria declared himself Boss of Bosses and tried to strike an alliance with Joe Aiello, the Unione Siciliana chief in Chicago in 1929, who was Capone's enemy. Masseria promised to get rid of Capone if Aiello would cede parts of Chicago to him. Aiello blew up at Masseria and chased him out of Chicago. In a complete about-face, Masseria then encouraged Capone in his fight against Aiello. But there is no evidence that Masseria provided any direct support for Capone.
Masseria arranged for the assassination of Gaspar Milazzo, Aiello's top ally in Detroit. Milazzo was from Castellemmarese del Golfo in Sicily, and all Castellemmarese regarded the murder as a stain on their honor. Maranzano, the top Castellemmarese in NY, slugged it out with Masseria. Capone initially sent money to Masseria but, according to Capone biographer Laurence Bergreen, "...he wisely heeded a warning from Masseria's enemies to avoid doing anything rash, such as sending gunmen. As a result, when the smoke cleared, Capone was on good terms with the survivors..."
Puzo also screwed up re. the Commission meeting. In real life, Charlie Luciano, in one of the most brilliant moves of his career, formed the Commission--and appointed Capone as chairman, and Joe Bonanno, successor to Maranzano, as secretary. Luciano positioned himself as a mere "member." He didn't have to declare himself as the top guy--everyone knew he was. His generousity to Capone and Bonanno was an attempt to bring them into the fold and make them more cooperative with other crime bosses. Though Luciano was a Sicilian, he welcomed anyone who could be a good partner. Jews like Meyer Lansky, Bugsy Siegel, Lepke Bucholter and Dutch Shultz, though they were not Commission members, sat with and counseled the Commission. Shultz was assassinated on Luciano's order after he failed to heed a Commission warning to stop planning the murder of special prosecutor Thomas E. Dewey.
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